The Master Parts Manual shows a plug (group 0.714) for the timing pointer hole. I don't see a plug for the circular hole on the driver's side. I have four bell housings in my garage; two of them have the circular hole on the driver's side and two don't. One with the mystery hole is a truck bell housing with a foot starter, and the other is from an unknown origin. The two without the circular hole are on late 261s.

I don't see an illustration of the timing hole and pointer, But here is a description of the timing procedure from the 1954 shop manual. .

Hoe this helps.

Hoyt