Denny, that's really interesting. The 261's that I have in the shop have the dowel, the 1/2" threaded hole beneath them that yours does, and a 1/8" NPT threaded hole to the left of the 1/2" threaded hole. The 1/8" NPT hole is not connected to the oil galley and drains directly into the oil sump.
IMHO, the only real bullet proof way to tell what you have will be to pull the plug from the dowel hole. Not sure why it was added unless the dowel was drilled out for some reason. When you pull the plug, if there isn't something (such as a dowel) pushed in to block the oil galley, then you are set for bypass filtration. If there is a dowel behind the plug pushed in to block the oil galley, then you are set for full flow. Maybe the dowel was leaking and the plug was inserted to stop the leak? Regardless, I'd want to check to see what, if anything, was behind the plug.
And as Roy Rogers points out, a full flow should have 1/2" lines...but I wouldn't trust the size of the current lines to be the determining factor. I'd check behind the plug.
Matt
