This sure isn't any gasket I've ever seen on a B18 / B20 or B21 / B23 motor, automotive or marine. Did B16 motors have a valve tappet inspection cover? If so, this is probably it. It's new in original packaging and yours for the cost of postage.

Volvo built a 2.1 and 2.3 liter engine simultaneously so I figure it's unlikely they were cast as two different blocks. I know for marine use the AQ125 was a 2.1 liter and the AQ145 had the 96 mm bore. Later the 2.1 liter engine was dropped and 2.3 displacement became the "base" or single-...