[Arran]

I think that the main issue with the grilles in the Baby Grand style Philco 20s was the plywood that they chose for the front panel. I noticed on several repair threads for those things that they used a plywood with a thick single core laminated by two thinner veneers on the inside and the outside. I don't know what species of wood was chosen for the core but regardless of what it was it's a law of wood working that a board is always weakest parallel to the grain and strongest perpendicular to it. This is why you have the veneers in a sheet of plywood laid 90 degrees to one another. When you are dealing with a fret work grill you are cutting holes in all directions so in some areas, some parallel, some perpendicular, some diagonal. so using a tight grained hardwood plywood, with lots of thinner laminations rather then thinker ones will give you the strongest grill.
Regards
Arran