Not sure about which stage I reached

LXJ, your description was one of the best I have seen. You incorporated every learning point into your effort to reach a MMO, and in my experience you have succeeded.

Congratulations! You just gave yourself a Beautiful gift that will always be with you.

But once they built up to a strong wave of contractions my entire body felt to be contracting, and I was moaning so much at a point that I felt even a slight short of breath. But I didn't think I had any orgasm yet although this was definitely enjoyable. Such status lasted for like an hour before I had to stop because my leg muscles became quite sore as I was lying on my back with my legs up and feet sitting flat on the bed. ... I was wondering if someone knows which stage I have reached in this session. Was what I experienced the so-called P-wave?

It was much more than a P-wave my fellow traveler, what you experienced was a full body orgasm (see What's an Orgasm?), perhaps even a Super-O (see the definition), so Congratulations on the event.

So was this still involuntary contraction if I can knowingly trigger them?

Yes, you were experiencing orgasmic contractions but I'll to bet you weren't using 'force of will' to do so but rather using the imaginative power of your fantasy and your intent to trigger further contractions.

Should I add some voluntary contractions going forward?

This is a bit tricky to answer, trying to add contractions when you are already in the midst of orgasm may kick you out of the "zoneros" but during lulls in an Anerosession doing a few voluntary contractions can 'restart' the involuntaries. This is a balancing act you need to experiment with as your practice moves forward.

Should I pay particular attention to the prostate now that i know where it is?

Certainly you need to be cognizant of all the pleasurable sensations your body is producing, not just those emanating from your prostate. However, your prostate is the core generator here so frequent focus back on it is important. The metaphor I'd use is like a relaxing Sunday drive in the country, you look around frequently enjoying the scenery but your eyes need to come back to the road regularly so you remain on the road.

I finally got some involuntary contractions but am not sure how to go from there to the rumored Super-O.

IMHO, you just had your first Super-O. Super-O's come in many forms, so please do NOT discount your experience as something less than what it was. I agree with @m-bc, you successfully incorporated important aspects of practice such as 1.) Being aroused going into your session, 2.) Being relaxed, both physically and mentally, 3.) Being in a positive mind state, 4.) Using fantasy imagery to heighten arousal, 5.) You effectively "Just Let Go !". Don't concern yourself about stages, all in all, you are already there my friend, continue with what you are doing and it will get even better.
Good Vibes to You !

LXJ, your description portrays EXACTLY what I have been experiencing after about a month of weekly sessions with my Helix Syn, but have not been able to put into words as eloquent as yours - thank you! I use edible marijuana products (legal where I live) for my sessions and listen to stimulating audio, so I have never been sure if what I have been feeling is a result of successful prostate massage or if it's entirely a drug induced "mental" response. (When I do a drug free session I have a somewhat similar result, but much milder).

RUMEL, I have mixed feelings about your belief that these are likely Super-O's - on the one hand I will be pleased at "succeeding", but on the other hand I am a little disappointed because, while pleasurable, the feelings that I have experienced (as described by LXJ above) do not seem as intense as what a traditional orgasm gives me. The advantage of my Aneros induced feelings though is of course the duration of them and the multiplicity of them. Can I expect intensity to increase?

ALL, I love this forum and the open sharing of learning experiences contributed by all - thanks!

RUMEL, I have mixed feelings about your belief that these are likely Super-O's - on the one hand I will be pleased at "succeeding", but on the other hand I am a little disappointed because, while pleasurable, the feelings that I have experienced (as described by LXJ above) do not seem as intense as what a traditional orgasm gives me. The advantage of my Aneros induced feelings though is of course the duration of them and the multiplicity of them.

I'd like you to re-read the definition of what a Super-O is, then also please read @Buster 's thread What exactly is a Super-O? to get a wider perspective of what a Super-O is in reality. It might help to also read What's an Orgasm?

The ejaculatory orgasm we men are most familiar with is usually very intense but lasts for a very short time, on the order of 10-15 seconds at the most. If we plot the area under the intensity and time elapsed curve and call this pleasure then it becomes quite apparent the total pleasure from prostate based non-ejaculatory orgasms (even when less intense than ejaculatory orgasms) far exceeds those 'crotch sneezes' we are most familiar with. Please note I'm talking about solo play here, partnered play is a whole different realm due to emotional engagement.

As has been so often stated, each of us is a unique individual with unique responses in our Aneros journeys. You are the sole interpreter of your sensations and how you perceive and mentally interpret those sensations dictates the total intensity of the experience. As you are able to become more focused on the minutia of the sensations the apparent intensity will increase. You've already noticed this through the perception altering effects of cannabis intoxication.

Can I expect intensity to increase?

As has also been often stated, drop all expectations but keep hopeful desires stoking your arousal, this is when intensity may noticeably increase but don't expect it ! I've frequently read about members who have experienced amazingly intense sensations "...when I least expected..." it.

Many new Aneros users (I include myself in that group) fell into the trap of expecting their Aneros use to match the hyperbole they had read on the web from others. Some of that hyperbole can lead to frustrating myths which must then be overcome to find the truth in their own journey. I suggest you see Super-O Myths & Illusions regarding 'the Edge' and a couple of other myths. Also please read @neros thread The Myth of Super Orgasm
Good Vibes to You !