Without having removed the filter yet, annual is next month, I think I've solved the puzzle. The thread in the adapter is 3/4-16 and a 48110 filter with male threads will screw directly into it when the can filter is removed. The proper way to convert to the '108 or '109 filter is with 3/4-16 stud which was included in the service kit to convert. What probably was installed, yet to be confirmed, is a Lycoming stud, 25010628, which has 3/4-16 threads on one end and 13/16-16 on the other thus taking the '103 filter.
There is service information that says the bypass valve in the adapter may come loose and if loose be replaced by a plug that gets staked in. Not all adapters have a bypass valve and those that don't or have been modified by removing and plugging the hole must use a '108 or '109 filter as only those have a bypass valve integral to the filter. None of the other filters have a bypass but all have anti drain-back, so, at the moment, my '103, installed on an adapter with a bypass is ok, if not exactly correct. I'll confirm when I do the next change where if the adapter bypass is Ok, I'll use my remaining '103 filter and order the correct stud.
Discussing this with another mechanic I uncovered a misunderstanding about anti drain back valves: They're not to keep the filter full as it's free to empty through the center stud depending upon the orientation but to keep the oil pump full. It shuts the IN line keeping oil in the line thus ensuring the oil pump doesn't empty and remains primed. An engine should not sit with the filter removed for any length of time with filter removed.