This is the inside caseback of my 1966 5513 that I posted earlier in the thread to say hello. It is the weird Rolex VI.66 mark which shows the 6th quarter of the year (VI), this is commonly thought to have been a mistake at Rolex I think and is quite a normal feature. Your 5512 might also say 5513 in the caseback but that is normal as well, as they used the same parts for these. Fortunately for me I am not really bothered about the birth year thing as I predate this watch. I am almost tempted to say "fancy a swap"!