Conservidave said:
so I'm leaning to the factory mag/spring starting to weaken enough to lose that critical timing.
I haven't spent a lot of time with 1911s, but...
I don't think "timing" is a critical factor for an empty mag (but there are things I don't understand, and I may be wrong.) Timing problems from weak springs tend to be a factor when the spring isn't lifting the next round quickly enough to be caught by the slide as it moves forward. That IS crtical, and noticed when someone installs a heavier recoil spring.
Do you have more than one mag? (If you don't, you really SHOULD have more than one!) If you don't, try to borrow one. A different mag would tell you whether its a mag issue, or not. If you have two mags, one that works and one that doesn't, disassembling the two mags -- the good and the bad (keeping the parts separate) -- would let you see if anything is noticeably different. Be warned: very new mag springs are always much longer than ones that have been used; springs take a set quickly, and will shorten, but still work properly.
There's a small chance something has happened to the slide stop lever, but if a known good mag isn't affected, it's not the slide stop.
It might be that the problem mag spring isn't strong enough to push the follower up flatly or properly when the rounds are all gone -- but the fact that the mag worked properly for the other rounds suggests the spring MIGHT be OK.. (Having the mag spring in wrong could give you the same problem with the last round, but that would generally cause feeding problems, too.)
Note: racking the slide by hand isn't going to give the same "motion" as letting the slide do it after the last round is fired.