MyMPx.org - Chipset & firmware differences (S7x variants...?)

In my search for firmware for my Glacier clone (2" screen, 1GB (after I removed memory hack), I'm getting confused. My player's original firmware was for ACU7515_S75 (player settings described it as 2006/01/29 9.1.51 ACU7515_S75 2006/10/21). The player stopped working (Had hourglass but would not boot) so I decided (retrospectively maybe wrong decision) to replace firmware with one downloaded from Uxcell. And, along with 000's of others I have a semi-dead player :(
After lots of trial & error I found a firmware (2006/09/12 9.0.48 ACT2097_S75 2006/06/12) which at least gives some screen activity (flickering text & some "random"patterns, but no photo or video.)Navigation is rather blind but functions seem to work.

Now, my question: Is it a coincidence that there is _S75 after the chip numbers in both the original & the "best I have found so far" firmware versions? Should I be looking just for ACU7515_S75 or is ACU7515 alone likely to be ok?

A member of this forum had access to some 9.1.51 firmware packs but advised me that the one which looked to be for _S75 had been superceded by one which had just ACU7515 (which did not work). This has confused me as I was under impression that _S7x after a chip number indicated a different version & therefore a different firmware. I've asked him to let me have the previous version but he tells me its been superceded by the version he sent (which looked to be a ACU7515 without the _S75).

Can anyone help/enlighten me on this so I can keep searching for this elusive firmware as efficiently as possible?

Thanks