I'm pretty sure klmz was the person who merged VBA-rr with VBA-m to make VBA 24m. VBA versions not labeled m stem from older VBA-rr versions. In my experience, those older VBA-rr versions have more emulation glitches than VBA-24m (noticeable in games like Mario & Luigi Superstar Saga, or some gameboy games such as Alleyway(where you can see a graphical glitch with the scrolling in older VBA versions))*.
I'm not sure how those older VBA-rr versions (23 and 24 not labeled m) have improved lately in that they fixed some emulation glitches. Sorry, but I don't know why these versions are maintained. And my post is almost more speculative than informative.. I mean, I can only point out that I saw many emulation glitches in older versions get fixed in 24m but I don't know if 24m is actually accurate emulating lag behavior.
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* I have made a post some years ago somewhere, where I point out which version of VBA fixed which emulation glitch. EDIT
here but I don't know if "24 debug" refers to regular 24 or to 24m in an early stage...