"Also note that the agency has changed their minds on this several times over the years, and might do so yet again."
While Congress foolishly granted the ATF with all the functions of government (the ability to write, enforce, and rule upon gun laws), they do typically do a decent job of putting out letters indicating their current state of enforcement. The only letter I've seen on these stocks (2010 or so, IIRC) specifically states original stocks or "Identical" reproductions (ironically the same term used to define eligibility for 922r requirements, which encompass firearms configurations very different in appears from what may be imported
) are acceptable.
I purchased a C96 recently which has the stock slot, and Centerfire Systems has a very nice looking reproduction for sale, so I asked this question over on THR a while back. No one seemed to know there, either, but were content to
assume the ATF had seen fit to infringe upon us. The way I figure it, this question has been asked/addressed so many times there is little potential harm* in asking yet again.
*It is really dumb to ask the ATF questions carelessly especially vague ones, since each is an opportunity for them to clarify undefined positions against us. While a certain unaddressed action
may bring their hammer down upon you in certain situations (like if you are a drug kingpin), if they've issued a letter on the action and see you are in violation they
will take notice.
TCB