[Arran]

...Mine is actually one of the 16B tombstones with a Code 121 chassis in it. AFAICT it was this way from the factory. It has a fully finished wood plug to reduce the size of the tuning shaft opening (earlier thread) - and re-drilled chassis bolts holes that look to me like they were professionally done...

By the time Philco introduced the shouldered 16B cabinet (around January 1935), the old 5-band chassis had been out of production for three months or so, and they made over 18,000 of the "peaked" cabinets. I would say 95 to 99% of those had the newer 4-band chassis. So once the shouldered cabinet went into production, Philco would have only had 4-band chassis in production.

The point? Any competent, skilled woodworker could have made the plug for that larger hole for the tuning shaft, as well as redrilled mounting holes in the bottom. I do not believe this was done at the factory, but by someone after the fact who had a 16-121 chassis and a 16-125 shouldered cabinet, and was skilled enough to perform a shotgun wedding of the two.

Finally, if it had been done at the factory (and it was not, in my opinion), they would have tried harder to match the surrounding veneer. An at-home "weekend warrior"? Not as likely to try to make the plug match the veneer.

Ron;
  As most of us know Philco had it's own cabinet plant, so they could, and often did, make cabinets to order. Because of this there would be no reason to go back and plug control shaft holes, or redrill a cabinet bottom to fit a different chassis, if they had left over chassis then they would have just made a short production run of cabinets with the right sized holes in the right places.
Regards
Arran