Hello and thank you for your answers, all of them very helpful for a Montesa 'old newbie' like myself. Dave, your Mk4b is beautiful!
Regarding the black engine,TopCat very correctly observed that it could be a 248 bottom case and 247 top end, but I am editing my answer as I found (call me slow, but I noticed it only this afternoon, as the bike is 30 miles away from my home and checking is not easy....) that the engine and chassis numbers match, so the engine and chassis were born together and put together at the time of the first assembly of the bike. It is also clear that this is a true 247, as its chassis/engine numbers are 21M2**** while the Montesa: VIVA book by mr Herreros show that the 248 had numbers beginning fron 21M51***.
It would be useful to know if anyone has a Montesa chassis numbers list so to check when this unusual late-series 247 was produced. Sorry for the editing, but I thought that it was not correct to keep here some wrong remarks that I made before I found the reality of the matching-numbers chassis.
All the best
Stefano