Difference between SST 120 and XR2 powerhead?

I'm in the search of a decent SST 120 powerhead and have found a couple now that I'm looking into (one of them is supposedly a XR2)...

I know that some powerheads here in Norway are mixed parts from XR2 and SST 120 and have heard that it is mainly the flywheel, ignition (?) and starter that is the real difference from the leftie SST120 and XR2 but how can I tell?

Is the serial# also the same?

Thanks,
Lars

just check the parts books

I don't have SST120 and XR2 parts books...
Do you know where these might be available?

Best regards,
Lars

Of course I found the online part lists at Mercury and made some comparisons but it seems like almost all parts have different numbers...

Might that be by Merc to block you on interchange SST and XR2 parts (even if the parts itself are the same)?

Confusing and I have a hard time finding out what I'm looking at here...