Wow I have no idea what you have that base matches nothing I've seen or have in any of my books.
To me it looks like somebody has joined two mowers into one.
The base looks like a Utility base something like a Model 550 1968 right through to a 1971-72 Utility model 71VH. However somebody has cut the axle brackets off and welded a Rotomos axles on.Also used the Rotomos handles and tank but then put a series 70 engine on it.Could you post up the engine number of the engine on the base I'd just like to know what year it was made.
That other engine you have looks like a late model 5 engine.Just curious as to why the head appears to be round the wrong way.
coolrussy believe me that is in no way a model 2a.
The one pictured below is a 2a.