Once upon a time when that language was federal law rather than MGL, the ATF Technical Branch would respond by letter to specific technical questions on whether a given method or item was acceptable for staying legal under US firearms code, or how a given item was classified under USC. Quite a few of these letters were made public by the entities that asked the question. If you search for something like 'ATF Technical Branch letters', you'll find examples. When the federal ban sunsetted, the DPRofMA incorporated the USC language, but did not specifically incorporate the previous decisions of the ATF tech branch as how to execute the MGLs. MA does not have an equivalent to the ATFs tech branch, and isn't making those sort of determinations - at least not in public. It is assumed that the ATF AWB determinations are still valid, and there's no MA case law that I'm aware of to contradict those determinations. That, I think, brings us up to where we are today. As an example of a related ATF tech branch letter that does mention ATF acceptable methods for permanently attaching a muzzle device:
I took a quick look, but didn't find any on stock pinning - I'm sure they're out there somewhere.
The ATF tech branch will still answer specific questions, but not AWB questions any more. No longer their problem. They have not always been consistent in their responses, either - see 'pistol braces'.