Shorrick, excuse me for loosing patience; my annoyance was not really with you, nor with anything you wrote.
Chaps – I did not start a new thread because I thought the diversion had come to a natural end.
But if anyone feels it is worth a new thread at this point, please do start one.
None. Feel free to report it for deletion.
And the relevance to a Trig transceiver is WHAT ?
Silvaire if you want to source Yak 52 hardware Aerostar is a lot easier to deal with and their stock is likely to be far newer than Russian stock. Many US-flying 52s are likely to be Aerostars to begin with.
That Garmin manuals site, http://static.garmincdn.com/pumac/ is very interesting. There is no directory listing enabled on the server, but various lists of likely filenames have appeared online in the past. Then, one day, google managed to get in and indexed up the whole lot! A lot of people helped themselves to it, with some quickly hacked-together scripts. One guy downloaded the whole lot, many GB, but when later I asked him for some manual he denied having done it 
Wow, that does seem to bring the answer but it isn’t exactly light reading. Thanks anyway, Jesse, it was my fault to overlook the Garmin bit.
Alweer hartelijk dank!
I tried searching for a description of that $PMRRC data but our alther friend G found very little. I learned that 9600 Bd must be used, and consulting the description of the Apollo SL40 I was struck by the similarity, all very nice but it leaves much to be found. Any pointers, please?
Have a look at Apendix B of the Garmin GTR-200 installation manual which is freely available from Garmin.
If y’all will excuse me for sticking to the original topic* : regarding
The TY-91 use NMEA input, with Apollo SL40 /Garmin VHF out format ($PMRRC).
I tried searching for a description of that $PMRRC data but our alther friend G found very little. I learned that 9600 Bd must be used, and consulting the description of the Apollo SL40 I was struck by the similarity, all very nice but it leaves much to be found. Any pointers, please?
*I would really have liked to see the “metric vs. imperial” discussion split off, it is so irrelevant here. If the 4-40 really annoys me I’ll run something~ hard and threaded through the receptacles and M3 it will be.
~ I think it is called a tap in English, though that word can also indicate the device to control water flow, itself called a “fawcett” or “faucett” in the US of A. Who said English is a simple easy language?
Ultranomad, thanks much for the info and offer. The link appears inoperative for me now, but I will try it again when I’m home. There are plenty of people with Yak 50s and 52s who I think might like the possibility to source airframe hardware. Now we know the real reason why people take Russian wives 
Actually now that I think about it one of the US sources for Yak stuff is IIRC married to a Czech lady who I imagine may speak Russian.
