What type VW motor do you have numbers on case halves read S1 12 401.101.101A?

You have a Type 4 motor--your bus is at least a 1972, right? A Volkswagen part number has three subgroups. I don't know what the S1 12 stuff is...could be the aluminum alloy VW used, or the mold this case came out of, or whatever. First subgroup: the car the part was originally used in. This one's weird because Volkswagen never built a car called a 401. I think this is probably a two-liter engine, which was introduced after the Type 4 was discontinued. But they can't put a "211" number on it, because the engine is a "Type 4" engine, so someone just pulled a number out of the air. Second subgroup: functional area. The first digit reports the "main product group"--1 for engines, 3 for transmissions...there are nine subgroups. The figure zero will never start this group. The last two digits report what item in the subgroup the part is--01 is the complete thing, 98 is a repair parts kit. 101 means it's the whole engine. Third subgroup: the actual part number. 101 is just the first one of these they made. Just call me the Fount of Useless Knowledge.