Engine block I.D. pictures

[svm99]

I have a couple of prewar 3 3/16 bore blocks with the keystone water passage. One of the blocks has an extra tapped oil hole into the oil passage at the rear of the block. The photo of the 59L block in previous posts show the boss but the hole is untapped. My question is how was the this used in the system? Did it allow for a full flow oil filter or oil cooler connection. I'm adding a "59 type oil filter" and am wondering if I could plumb it with this hole. The 3 tapped holes on the bell housing area are all into the same passage way which is confusing me.

Any ideas? I'll try to post a pic in the next day or 2.

Thanks Paul J.