Don't believe there is any other source to find the original engine # for a car other than through the Archive. With the chassis # Lars Gerdin at the Volvo Museum and Historical Archive can give you all of the info including the engine serial # as in the chart I posted earlier.
On a B18 the first of the two milled pads on the LH side of the engine below the head is stamped with the engine type # (the tune). The engine # is on the second milled pad. If I remember correctly the B20 numbers are in the same place.
You probably know, but many people new to old cars don't know exactly what a numbers matching car means. In this case all the #s on the chart including the engine # match the plate on the pedal box and the engine and transmission.
If all of the #s are the same it is a numbers matching car as it left the factory, which is rare. Many of the B18 powered cars that survive have had a B20 engine swap.
It is easy to contact Lars Gerdin @ the Archives:
lars.gerdin@consultant.volvo.com