Hello Ivan,
I'm a newbie on here (although I have owned 4 RRCs over the last 20 years) so I wouldn't want to post anything too controvertial. However, from your description, if you lived in the UK, I would advise you to be very wary of a car with the appearance of tampering to the chassis number unless there was a good story to explain it.
Living in Belgrade, as you do, I would guess that this is less of a problem.
Are you talking about cars that have been imported from other european countries?
By the way, none of my RRCs have had a chassis number on the chassis!!!
Cheers
Chris