It is custom labeled R2R DAC network, so in theory armed with info how R2R networks should look like, it is possible to determine the resistances.
So between any two data pins, you should find 2R+R+2R.
Between output and most significant data bit (D7, LPT pin #9), there should be 2R.
Between ground and least significant data bit (D0, LPT pin #2), there should be 2R+2R.
Or just buy one of these, with say 10k and 20k resistances :https://www.bourns.com/pdfs/r2r.pdf
Although, the Covox patents say the resistances should be R=100k, 2R=200k, but to attenuate the audio from 5V levels to audio levels, the output to ground resistance is 15k. Capacitance about 0.005 microfarads, so I suspect 4.7nF, resulting into about 3000 Hz low pass filter bandwith.
OK, it does not completely add up. 15k in parallel with 100k R2R output impedance is about 13k, and 13k with 4.7nF is already 2600 Hz low pass filter.
Also a slight point to note is that the in the patent schematics, the R2R network's least significant bit's side is not connected to GND through 2R. In theory it should, in practice it most likely makes little difference regarding the sound quality.