First off, I've not worked on an A series since the 70's, in my bugeye
I thought it looked like front, left corner above the pan ahead of the pump?
there was also mention of racers back in the day putting 1098 heads on 948 blocks
I'll see if I can relocate the article
edit try this page
https://www.mgexp.com/article/bmc-a-series-engine-codes.313
not the one I saw earlier, but looks helpful
Minimania might have something, but I can't get on their site today
It seems that for the early engines, they used a system where the number was the size group and letters were sub groups
so 8xx was anything in the 800-900cc range, and 9xx was the 900-1000 group