This happened in 1953, when I was stationed at a small overseas unit in the army. Remember in those days that any MM activity was considered criminal and in the forces was likely to cause imprisonment and dishonourable discharge.
I had been a very active wanker since about 10 or 11 and had many schoolboy MM experiences up to the age of 15. After that, it was just solo. A few months earlier, this good-looking lad arrived in the squadron and I did feel attracted somewhat to him. I was due to leave for demobilisation the following day and the custom was that I stood a couple of rounds of drinks in the canteen. The evening progressed and the elation of going home after a tour of eighteen months perhaps made me less inhibited. I had to go out to the latrines to relieve myself and found myself standing next to this boy, alone. I asked him outright whether I could hold his cock while he peed (daring for those days!). He agreed and I gently took hold of it, directing his flow into the urinal. He did the same to me. When he had finished, I tentatively stroked his dick slightly and I could feel it start to react. Then he invited me for a proper wank.
We went round to the sergeants' latrines (we knew there were no sergeants in camp that night) and dropped our trousers and had 10 minutes of undiluted joy, handling each other's cocks, just cumming onto the concrete floor.
The following morning, I was going the rounds saying my farewells when I shook hands with this guy, with more than a twinkle in our eyes. He told me that he wished he had known months before that I wanked as he had always liked me and would have wanted to have many mutual wanks. I replied that I too, would have wished it, but it was just as well we didn't know, because it would have been sure that we would have been found out and got into very serious trouble. He agreed.
I have had many, many solo wanks based on the memory of that single act 52 years ago: the last one was last night!