This one's simple enough - if it has B20 cast into the drivers side of the block (assuming your not in the UK) up near the cylinder head it's a B20. There are many (if not most) parts that interchange between the Penta AQ engines - often used for performance upgrades or less expensive parts (steel timing gears come to mind). As for the head, they came in three basic varients and are separated by some differences in the casting (search for pictures of this one). There are two NA heads (late and early B20B sometimes the early is called the B20A...but that is a little foggy for me) and two injection heads (low compression B20F and high compression B20E). You can tell the difference as the compression ratio of the E head makes it pretty skinny compared to the F. Most North American B20's are either B20B or B20F's...I happen to have a late B20B head that is the same as a B20F (larger valves) but no injector ports.

So don't wory about the distributor - I find that many old Volvo's have parts from lots of different models on them, originality is less important than running in this world (such a relief).

Craig