Sadly if the radiator shroud YG2S4 xxx stamping and the xxx 2/06 gutter stamping is unreadable or missing the car can not be confirmed as a genuine Mk2 S.

This would affect the value, however it would be more than a Mini K replica but less than a Genuine Mk2 S.

If you remove all the mechanical components, body hardware and trim from a 1969 MK II Cooper S body shell so it is completely bare, and compare it to a non-Cooper S body shell (Mini K or Mini Matic for example) in the same state you will find a number of areas where they differ. By closely examining those areas to see if they appear to be factory original and not created subsequently you can form a view as to whether a body claimed to be Cooper S is more likely than not to be genuine. It's a bit like the steps to identify yourself when applying for a bank account or passport. You submit multiple items of "evidence" that, when viewed together, allows the authority to determine that it is more likely than not that you are who you say you are.

The areas to examine include the location of the right hand tank aperture, breather tube tabs location and method of fixing in the boot, presence of a hole in the boot floor for breather tube exit, fuel pump breather tube hole, location and method of fixing heater mounting brackets, and position and method of fixing the angled brace behind the grille.

Someone with knowledge, motivation, skill, time and appropriate welding and stamping tools could produce a Cooper S body shell from a non-S donor that would be difficult, if not impossible, to tell was not factory original. Reproducing accurate spot welds done during sequential assembly of body panels on the production line is extremely difficult in some areas without removing and reattaching adjacent panels. This work would be costly so for most people not worth the investment. That's why some knowledgeable Cooper S enthusiasts can usually spot a replica upon close examination.

cooperess - you should try to get the best possible images of the stamp locations mentioned as sometimes a partial imprint can provide sufficient evidence that the shell is Cooper S, even if the numeric body number or car number part is incomplete or undecipherable. I recommend gtogreen1969 as a subject matter expert in such things should you want an opinion.

It's not just the actual stamped characters you can read but also their size and orientation on the gutter/panel that can help determine original or not.

Why not just enjoy driving the car that is I assume otherwise specked as the road rocket many of us enjoy. It shouldn’t be about a a theoretical value.

Totally agree with "minijcar"...unless it IS about a "theoretical value".

If you want to sell it as a MK2 S body then uncovering the chassis and body stamps would be beneficial. Also correct spot welded tabs on the front bulkhead and rear bulkhead/parcel shelf. I don't think they can be spot welded to an assembled shell.

They have single sided spot welders out there

Oh wow. How do I verify those?

gtogreen1969 is arguably the most knowledgeable on font type, font size, location and orientation. I think he would be happy to advise (confidentially) if you provide the best images you can get.