Thermostat T694B replacing with Honeywell SuitePRO TB6575A1000

I am replacing a Thermostat T694B with a Honeywell SuitePRO TB6575A1000 in an apartment building with switchover heating/cooling, so presumably two-pipe system.

Button-wiring mapping:
Lo: 1
Med: 2
Hi: 3
Summer: 4
Winter: 5

I’ve attached a photo of the T694B wiring diagram but I’m not sure it matches how my thermostat is wired.

Only have three wires from old thermostat to junction box: two black (connected to 2 and 3) and one yellow.

2 and ground have 120V when Med button pushed. Otherwise is 49V.

3 and ground have 120V when Hi button pushed. Otherwise is 49V. When black wire is disconnected from terminal 3, it measure 44V.

L1 and ground = 120V.
L2 (is jumpered with L1) and ground = 120V

Yellow wire from thermostat (for valve?) connected in junction box to both black wire labeled "120" and white wire (I have uncapped them in photo). Black wire labeled "120" and ground = 120V. I can’t remember for certain but think the black "120" wire and the white wire it’s connected to (along with yellow) come out of the same white Romex-type cable (middle of the three in picture).

Power flows from black "120" wire in wall down yellow wire in thermostat to 2 or 3 depending on which button is pressed to control fan speed. Also, the 120 goes back out via the white wire. Correct?

The other two white wires in the junction box are connected and capped (orange cap in photo). Why doesn’t the old thermostat have a neutral wire attached to it?

Given schematic of new thermostat, it looks like Gm and Gh connect to the black wires currently wired to terminals 2 and 3, respectively (fan speed in current thermostat).

Assuming that is correct, how to wire up the rest: L (hot), W/Y, and N in new thermostat?

Do I connect the wall "120" black wire to new thermostat terminals L, and W/Y?

Any help and suggestions (and explanation of what is going on in my thermostat) would be awesome and much appreciated!