Dear Tom;
All 59 261s can be "full flow", It's the position of the moveable pin that tells if it had a filter or not. The 1954 model is the only one with a 'by-pass' filter option.**
Someone may have removed this pin if there's a hole W/O threads on the 'drivers side' etc.
Pontiac ran out of engines & the Chevy truck (261) engines were then used etc.
Waiting to see your photos.
**Micky Mouse could add one though.
