birchbark,
3B's reference to "high-pressure" engines (some 235s and all 261) as compared to "low-pressure" engines (all 216s and some 235s) is far better than referring to "full pressure" vs "no pressure". One reason is that Chevrolet literature referred to low-pressure engines as being "full-pressure" (with respect to main and cam bearings).
The canister by-pass oil filter was an option and the same canister could be used on all 216 and 235 engines (and on 261 engines through 1957).
What casting numbers do you have and where on the engine did you find each of them?
Here is some information on where to find the various codes on a low-pressure engine.
