It looks like there is a casting 'bulge' there, but no dowel/pin. The dowel would be obvious in the bulge. It would take a ton of paint to cover it up. The dowel was added starting in 1959, so if your engine is a '58, it wouldn't have one.
The odd thing is that you have both 1/8" and 1/2" ports. The dowel engines had both, but I'm pretty sure that the '58 and earlier ones just had the 1/8".
You might have a Canadian Pontiac 261. I have read on this website, but have not seen for myself, that the (or at least some) Canadian 261's had the casting bulge but no dowel. They were set up for bypass only, or no filter, and have both the 1/2" and 1/8" ports. If that's the case, others can confirm but I think you'd use the 1/8" port for the bypass filter and plug the 1/2". Also be sure to use a bypass filter that has the 1/16" restriction in it, just like on a 235, to avoid starving the bearings of oil.
No big deal if you can't do full filtration. Bypass is fine.
Matt
