Hmmm may be my first post being able to help a little here.
I have just dismantled a Genuine EG6 SIR-S. I have a lot of the parts from it left still. Keyless entry setup, interior...etc..etc
Most SIR-S's I can tell you will be on a "K" plate, they were mostly, if not all made in 1993. As far as can be told any way, from what is known about them.
As the member above said, they came in Black, Red and Green. Some speculate that there were a few in Captive Blue Pearl as well, and there are people in the US that say they've seen them or own one.
Also its a common misconception that they ALL came with TCS/Traction Control, they didn't. Even some SIR's and SIR2's came with traction control. It was an option, just like the JDM parking pole was too, and a fair few others.....
They came with plain dark Charcoal interior, it looks almost black and there are no lines or patterns anywhere on the interior (some out there say they've had a patterned type interior as well)
The doors usually had DOHC towards the front wings in large lettering, unlike the normal SIR2 that had it towards the rear arches/quarters. If you look closely at the paint some times you can see outlines or discolorations where the old stickers may have been, this will be a sure indicator as to weather it was a true SIR-S.
Another thing I can also tell you about them, and this is gospel as I doubt many people have had this experience - the actual shell itself, is VERY lightweight, I have the exact weight of the shell some where from where it was scrapped as a bare shell. And for some reason it was slightly lighter than 2 EF shell's and another EG shell that I've scrapped due to damage and severe rust.
The doors, possible the same on all SIR models, but 100% the case with the SIR-S are extremely lightweight. There are no door bars in there, and you can physically feel a massive difference when holding a normal EG VTi door (or other) in one hand and the SIR-S door in the other - the difference is unreal.
OK now down to things like Keyless entry.....
It's common for people to think that Keyless entry only ever came on the SIR-S, that's wrong, it also came on the 20Th Anniversary models too, and also the Chuubu EG6, but again, I'm sure there are some SIR-S's and 20th's out there without them too. When these EG's were new im sure it was more than possible to request them without the option.
Now I find it very difficult to believe that the guy you bought it from has - changed the keyless entry, removed the decals AND replaced the interior with a possible non SIR-S interior.
Doesn't make a lot of sense, either way I can offer help if you want to put it back to SIR-S spec.
Also..... the SIR-S isn't "the" rarest EG model ever built either