jon20013,
Again, I don't have the current A2.4, so I don't know whether it's been changed, but the M you referred to in your post is to be shown outside the rectangle, not inside it. But, as you stated, it does indicate machining, and is used primarily for plug and slot welds. Also, a joint with a spacer, which I think Brian was referring to as an insert, is designated with, or used to be designated with an M inside a rectangular box as well, but the two different scenarios were/are easily distinguishable because of the differences in the two welding symbols.
I have a couple old A2.4's, one from 1979 and one from 1986 that I refer to from time to time. Keeping those old publications comes in handy sometimes.