To address the items you mentioned. Yes I agree that they (the oil filters) arrived at the engine plant prepainted. This made things easier on the workers and plants especially on those applications where they could not be installed on the engine prior to painting like FE applications. Do recall a number of small block examples that have turned up over the years at shows and swap meets, removed from cars, with very visible two coats of paint (one peeling from the other or the visible change in texture and slight color where the surfaces closer to where they were attached to the engine as if the painter did take to the to cover all the surface since it was unneeded.
As to the paint instructions that made up one page of the engine assembly guides the text is pretty much boiler plate text copied from one year to another with few changes. One was the addition of the posted vacuum switches on the small block sheets beginning in 67. The text was sometimes specific (like cover 1” of the nipple for the heater hose) while in others (example oil filter) were more general worded to keep clear of paint from the mating surfaces of the oil filter. I see this being similar to not getting paint on the rear edge/flange of the block where the workers accomplished this by installing the engine plate and for manual transmissions on some applications the bell housing also and to keep paint off the flywheel and clutch assembly by installing those also. Don’t think you could apply enough paint to glue the oil filter gasket to both the block and the filter all the way around and given the small gap between the two unless you really tried and the films don’t indicate the painters had the time nor focused on that area any more than any other part of the engine during painting.
Realize that you may have not seen either of these or others but wanted to offer the following examples of the filters in place while the engines were being painted from factory films.