Old Race 340 Teardown

I believe your block is an X block. Typically the X is cast in the front right and left rear in the bell housing area. Yours has the left rear X as you pointed out and is visible in post #1. Also the dipstick boss comes undrilled and some people drill them and install a dipstick. Yours remains undrilled. Also the front of the engine in the timing chain area has more material extended from the top left down on an angle towards the opposite side. Yours has that as shown in post # 15.

It could be your block had some machining cleanup or lightening that removed the front right X and the other cast in numbers on the sides.