I know this is old, but I think this thread is very informative. I somehow ended up reading the entire thing and the A4-bit prompted me to have a look at my own MMX chips to see what the deal could be with these markings and their supposedly better overclock.
I don't have a testbench available at this time, but I did write down the markings of all the MMX chips I had stored in a couple trays and noticed a few things that might be of interest.
What I wanted to know, is if there was anything peculiar about the "MALAY" marking or the "A4", and apparently all the ones with the MALAY marking were made in a fab in Malay (big surprise there). The line below the MALAY line starts with an "L", which is the fab's code, and the ones with "A4" start with a "C" (indicating these chips were manufactured in the USA).
I found a webpage with information about the Pentiums here, it lists the fabs where any particular chip is produced.
Now I know the sample rate of only 3 A4 chips isn't much, but at first I thought perhaps these chips were better overclockers because of a later production date?
But it seems the A4 MMX's might simple be of better manufacture, which might make these better overclockers. Otoh, my oldest A4 chip (I only checked all my 200's and 233's and skipped the CPGA and 166MHz models) had a production date of 721 (1997, week 21, Pentium MMX 200) and the newest 838 (1998 week 38, Pentium MMX 233), while the oldest MALAY chip I had was from 726 (1997 week 26) and the most recent one was apparently produced in 037 (2000 week 37!). My 3 most recent chips also had a different font compared to all the other chips from either factory, dunno what the story is with that.
These 3 chips (date codes of the other 2 are 907 and 913) were all 3 made in the MALAY plant and have the same font used on all my Tillamooks (which apparently also were made in the Malay plant).
The superchip from Mystery has a date code of 818 (C818, the C is the plant code btw. Malay uses L), which may have been a more recent production date.
In theory, newer chips should be better overclockers.
Here's the list of all the MMX chips I checked (200MHz and 233MHz only)
Apparently all MALAY chips were made in the L plant and all A4 chips were made in C plant (perhaps I missed a single chip, but all of the chips I had my eyes on, followed this simple rule)
233 L907 (Tillamook font)
233 L037 (Tillamook font)
233 L913 (Tillamook font)