That is one fine cabinet restoration. Congratulations on a beautiful job.
The bias cell method was used in many cheap sets of the thirties to save money on parts needed to create bias voltage. I guess bean counters were plentiful in those days too. It surprises me that it is used in this model as I don’t recall ever seeing it used in RCA sets but in many sets sold by Airline and other catalog brands.
I’m curious what your modifications were in replacing the bias cell. My method of replacement is the same as in the original circuit. Take the bias cell and go shopping for 1 ½ volt button cell watch batteries which can be found in various diameters and thickness. Look for one that will fit the cell cup. I use # A76 or LR44 which fits the cup nicely. To replace it start by bending back the soft metal (zinc I believe) edges of the cup and digging out the dried cell. You’ll find it to be dried and crumbling. Clean the cup inside and out with a small piece of 4-0 steel wool until shiny and drop the battery in with the button facing up. I pack the cup with tin foil for better contact and to help center the battery. Then press the edges back down with a small screw driver tip to hold it in place being careful not to short the edges to the center button. The outer case of the battery is positive as is the cup of the bias cell. The center is the negative contact and you will usually have to adjust the clip to make better contact once the cell is put back in the wire holder. I have sets are still working fine after 15 years using this method.
I also followed your Firestone restoration as well and it is beautiful. I have a deco Bosch console that needs veneer on top. I am curious about the glue and thinning ratios that were used in applying the veneer. I want to try your method of using heat to set the glue.
Keep up the good work.