Man, I should keep hold o' that chip. Saw one for over £70
Anyway. Saw this one:

- Screenshot 2022-06-16 000613.jpg (74 KiB) Viewed 1392 times
Turns out it was the one used on my old atari bought in '95:

- 20190525_203138.jpg (560.99 KiB) Viewed 1392 times
Could have even been my chip since I sold him my old machine.
I know this is compatible with a 4MB 520STFM so lose nothing by testing it.
Then I'm done. This or the original goes back in and I crack on with music.
US Atari 520 STFM (C070523-001 REV.D1) , Exxos PSU, 4MB Marpet Upgrade, Exxos 6 Chip TOS 1.04 USA, EmuTOS 1.0.1, Ultrasatan, PP Driver, Original Floppy Disk Drive, NEC Multisync EA193Mi Monitor, Star LC-10c Printer, iMP C100110-001 DMA.
https://thecosmicpuppets.bandcamp.com/releases My released music so far...