Identification of diode in a form I've never seen before

[magic]

If i put the diodes back to measure voltage drop, what points on the board should I measure between to get the best reading? Just across each leg of the mystery diode?

Directly across the diode. But it doesn't matter, copper is resistive so Ohm's law applies: voltage difference between the ends of a PCB track equals resistance (maybe a few mΩ) times current (maybe a few mA in this case).

Actually, don't even put it back but start with measuring in both directions using diode mode on the DMM. If it's a single diode you should see something around 600mV in one direction and out of range in the other. Multiple diodes is trickier, some DMMs may be unable to measure it.

[floobydust]

OK I updated the schematic in post #18, it more or less makes sense now.
The diff amp stage, a bit odd to have a trimpot there I assume it's for offset adjust.
The diode looks like a one diode part (one Vf) is all that is needed?

[magic]

Everybody who read Self knows that this is 2nd harmonic distortion adjustment

But trimming for zero output offset will probably get you close

[moothefish]

Looks like its just a single diode then, one measured 580mv and the other about 620. though when i first measured it it was reading almost 900, but subsequent readings were all about 620. Will put it back in circuit when I get time and see what it measures there