I suppose that depends on the manufacturer, but thinking about it the tail pipe is probably noticably different, however according to the book the only different tail pipe is for Belgium!!
The underseat tank was standard on 109's up to suffix B vehicles, then moved to the back, except for 6 cyl models and Station Wagons that always had it at the back - needless to say there will be exceptions!
Equally so on any Series motor (including rear tank).
Richard