welcome aboard Cirzoke, it was a standard 13mm maple shafts back in the day, as a matter of fact I was so used to 13mm shafts, some people still make them, some players still play with them, the LD Shafts got a bit thinner, some are real thin like Z shaft, 11.25mm, and the standard for LD Shafts is usually 12.5 or 12.75.
I switched to a Mezz LD shaft, but couldn't go for a real thin shaft, so I choosen the Alpha from Mezz, its the closest to your good ol' standard shaft but LD, and it is 12.75mm shaft, I can't feel that its a huge different from 13.mm to 12.75mm, so I got used to it rather quickly, and now I feel in love with it, it plays really well.
so my advice to you, if you're an old school coming back to pool and you were used to 13mm and wanting to try a LD, go for a 12.75mm, you won't notice big different, and I think you'll like it, and its taper is really beautiful shape, similar to the old maple shaft.
hope this helps, by the way from your avatar I think i seen you somewhere, care to share your name or location? thanks.