I happened to be peeking around Flickr and I found some pics of the 1956 stock sitting round with some 1959s. I've never seen a 1956 stock pic before, so this was quite a surprise and delight. The account also uploaded other pics of the old Underground.
I always thought the 56s were white, as compared to the silver of the 59s and 62s. The first time I remember seeing a 56 was at Uxbridge in 1973. My parents and I were travelling back to Hillingdon, and a Piccadilly entered the middle platform with a 4-car unit of 56 TS leading. It was tipping out and sheltering in Uxbridge sidings.
Only time will tell if I will ever take a journey on a 72, 73, 92, or D stock.