I believe that the "512K" actually refers to the capacity of the memory chips on the SIMM. The SIMM is layed out to accept up to 512Kbit PLCC-32 memory chips. Four 512Kbit memory chips gives one 256Kbytes of capacity.
The PLCC-32 package which the ROM/OTP EPROM/EEPROM/FLASH (non-volatile memory) chips come in have their pinout defined by a JEDEC standard.
The standard pinout for the PLCC-32 package changed significantly between the 512Kbit capacity non-volatile memory chips and the 1Mbit chips.
So, a SIMM which will take the 512Kbit and earlier has one pinout (connections) for the 512Kbit chips. A SIMM which will take 1Mbit and larger memory chips has a pinout with substantial differences from a board built for 512Kbit chips.
This is why we couldn't just take old SE/30 ROM SIMMs and replace the ROM chips on them with newly programmed Flash chips. The pinout on the SIMM for the PLCC-32 chips is wrong for the larger capacity memory chips. So Doug had to lay out a whole new board which would provide the proper pinout for the larger memory chips.