R90S engine number.

Just received a customer's 1976, R90S engine unit for an overhaul. It has the correct black barrels, etc,
but the only number stamped on the engine casing is next to the oil dipstick.
Rather than being the same number as the frame, this number is 7 digits long: 1815968.
I've searched BMbikes where all the R90S engine units seem to have the same number as the chassis.
Has anyone encountered this before or can offer an explanation? TIA.

Replacement engine (or block) at some point in its life is my guess.

The only number is the one stamped above the dipstick. As solidstate say's, probably replaced at some time .

Thanks for that, I do recall seeing a similar number on an old set of R100RS casings,
after the number I've noticed also that there is no BMW emblem.