RFTGm-II-Rb (KS-24019) GPS disciplining question

[tymm]

Hi,
I've got one of the Lucent RFTGm-II-Rb oscillators that I've used on and off for the past 10 years, without the matching XO unit and have just started to sort through getting it listening to a GPS unit (using an Oncore w/v2.2 firmware a number of years ago).  I'm attempting to connect the Oncore externally through J5 rather than mounting it internally, but from my understanding there should be little difference as the connectors are directly connected internally.  My GPS unit is running without a battery, so there wouldn't be any stored configuration to confuse things.

Possibly related, but I have Rev 1 of the hardware, with an Efratom rubidium unit, so guessing firmware may be older than other units.

Documentation I'm basing my approach on:

Pinouts & related information
https://www.mail-archive.com/time-nuts@lists.febo.com/msg01561.html

Super helpful write up from ZigmundRat on doing an internal GPS:
https://www.eevblog.com/forum/repair/need-gps-board-for-ks24361-aka-rftgu-o/msg3547273/#msg3547273

I'm using the beta Linux version of Lady Heather for checking, and the RFTG software from Ko4bb's site.

I've hooked up the Tx pin of the GPS to J5 pin 4 (GPS IN), and the PPS output to pins 8 and 9 (PPS IN / GPS PPS IN), and have programmed an ATTINY85 from the code given in the previous link, which I've done quick checks via Lady Heather is generating what seem to be correct output (Lady Heather is able to get time & satellite info from the output stream).

I've double verified that the J5 connector is directly wired to the internal GPS connector (no inverters or level shifting required).  And using the beta Linux version of Lady Heather seems to pull the correct time from the unit... but the "NO GPS" indicator stays lit, and the RFTG software indicates that the GPS time on the unit is unchanging, set to midnight 1980-01-06.

Wondering if anyone has tips on next steps trying to get it working -- I'm not sure if it's likely to be an issue with the GPS rollovers that have happened since the unit was deployed, a hardware issue, GPS receiver issue or something else.