Wow, thanks everybody for the replies. Yesterday evening, very late, I found an old German instruction manual for the YB20 with a scheme of the hydraulic system. In the center of the scheme I saw a code 3T75H-LTBS which sounded like an engine code to me. And Bingo!! This seemed to be it. And now that I just reconnected to this forum, after waking up in the morning, I found the replies here, confirming this engine code and also links to waterpump. You guys are great. I'll try to share some pictures of my monster forklift. It has had a very rough life, already before I bought it 25 years ago, but like I said, it has NEVER let me down, not even once. These pictures are 23 years old, by the way. But the YB20 is still going strong, well okay, apart from the waterpump.
In 1995 I bought a former cattle farm and transformed it into a horse boarding stable. We decided to make fences of thermo galvanized scaffolding pipes, and over the years we had lots of work to do in the fields. I stumbled across this "tracked forklift" and it was exactly what we needed at the time. We also use it as a "normal forklift", I mean on paved surface, when necessary. We will remove the waterpump today, compare it with the 3T75H-LTBS water pump (of which I have found exploded views drawings on Yanmar's own site) and compare it with the replacements that are being sold online. Then I will order a replacement and we will install it at soon as it arrives because nowadays we use the YB20 on a daily basis to shovel horse manure on a stack with a large snow plough we fitted over the forks. I'll post some more pictures later on. Cheers!