Have to say that when I read this the alarm bells went off. IRRC these 73's were never "engine- numbered" as such. They all started off as SL600's with M120 engines and subsequently converted. If this is indeed the case one can admit to the possibility of identity "confusion" or engine substitution further down the line. Without rock solid corroborative evidence I would be wary. Primary check on these bored and stroked engines would be to measure the cylinder displacement as I imagine there's not much visible evidence of any engine conversion. Has your friend checked the engine and chassis number with AMG?
There are a few things that give me the confidence it is what it purports to be...
Chassis number confirms it was originally an SL600
Chassis number check confirms the car was manufactured for the Brunei market.
I have been promised written evidence the car is a genuine ex SOB SL73
Condition of the car is commensurate with it being a SOB car, IE poorly cared for
Car has all the right cues that its an SL73
Has all the original handbooks that state SL73
Vendor is 100% trustworthy as is my contact. I can be certain neither of them is trying to pull my pants down
Car is still on its original tyres which helps corroborate the miles
The vendor is offering other ex SOB cars
Re speaking to the factory, I'm on the case with this.
A thorough inspection of the engine would be great and this will be done as part of the rebuild, but no time to do it now as danger is I will lose the opportunity. I'm taking a punt!