Old HP design schematics habits?

Hello ,
I repair or more precise restoring old HP Electronics Load , I have service manual with list of parts and full schematics.
And have problem.
See on picture, for example R173+R173+D34 they are in bold black frame.
These components are not listed in the parts list and on schematics are in bold black frame.
My question is why?
Were they added to the device additionally or were they removed as unnecessary?


I don't know why they are there
CV Prog is 0 +10V voltge by which set voltge on the load
VMON* is really voltage on the load 0 -10V this two voltage is summing on precise 0,1% resistor R105 , R117 and amplifier U13 LF412 drive big power MOSFET in the load so that VMON* =CV Prog. Simple and clear, I do not understand the meaning of D34 etc.
UNREG is voltage on gate first power MOSFET in the load (negative +6.5V for from close and full open MOSFET) , may be R171=91k and R173=9.1k but who knows, it doesn't make much sense to me.

You should be able to detect if the components are populated in the actual instrument, isn't it?

Possibly these are 'optional' components which are not always populated. If they ARE on the board then you might be able to see their values there. If not on the board, then simply ignore.

There is a position on the board for the parts, but they are not physically there in my piece, but there was no amplifier U13. I don't know what the fate of my piece was, but somebody was already digging in.

Question is, what is the function of this componets?

My first idea LF412 have input offset 1mV typ 3mV Max and maybe delete this ofset, but why fedback on unreg ( this is voltage on gate first power MOSFET in electronics load)

I just don't know what to do :-x