The 'standard' (for want of a better word) MK4128 is of a 2-chip piggybacked arrangement, two 64K-bit chips.
Pictured at [
here].
The top chip is slightly different to the bottom chip; RAS is pin 3 on one chip, pin 4 on the other chip.
In the 'same vein' as what Chuck suggested,
perhaps (repeat: perhaps) Mostek:
1. Produced batches of the RAS-variant-1 64K-bit chip (the 'top' chip), stamping them as MK4128 at the time (easier to do then, rather than later in the piggybacked arrangement); then
2. Tested those chips, discarding failures; then
3. Produced batches of the RAS-variant-2 64K-bit chip (the 'bottom' chip), leaving them unstamped; then
4. Tested those chips, discarding failures; then
5. Piggybacked the good chips (RAS-variant-1 on top of RAS-variant-2); then
6. Tested the piggybacked combination as 128K-bit (in case of bad soldering, etc.)
Of the discards of step #2, ones that failed only in the second half were sold to OEM's as 32K-bit chips, but unfortunately sold still labelled as 'MK4128'.
Mostek will have had to inform the OEM of which pin (3 or 4) was the RAS pin.