Very nice engine! Late '70s version, probably one of the first production YS engines. Note the machined 2-piece case versus the later sandcast, 1-piece case. Also, the fuel delivery system looks like the older design (before YS made engines and just manufactured the pump/regulator assembly for other engines). As for the pressure nipple in the muffler; my only thought is that they intended it to be sold for other engines as well. The muffler mounting/adaptor plate also hints at the aftermarket sales possibility.
Mounted in an MK Akromaster?
-Will
PS It probably had excellent performance - it is clearly Schnuerle ported based on the casting port 'bumps' seen on the front,rear and left side of the engine. Is it ringed or ABC? I'm guessing: ringed.