ID on a Mukuni Carb

Need help getting some more info/parts on this Mikuni Carb - the carb is marked 318 M1 then below that it looks like ISO with a cross symbol. Engine side measures 26mm and change. I have a feeling it was on a Yamaha maybe early 70s vintage. Seems to be missing 2 adjustment screws ...I want to rebuild it for use on a minibike build.