Is this circuit correct for controlling old 750mv HVAC with NEST
It was my fault. I did not check the Furnace Control board before posting the question. The system is 24VAC.
The C wire was not connected on my control board by the previous owner/installer because it was not needed.
Connected the C wire, and now I read 27v AC between R and C.
Tried connecting Nest, and that works too. Tested Fan, heating and cooling, all good.

(P.S. I feel stupid for not realizing the control board was on the bottom panel, I thought the board was hidden somewhere inside, and I was reluctant to unscrew anything major on the furnace)