[Ron Ramirez]

What was the cutoff year for using nitrocelluloid insulators?

Technically, 1937 - the 37-84 was identical to the 84 except for cabinet and using octal tubes instead of the old standard base tubes. The 37-84's coils were identical to the 84, and also use the film insulation. The 37-84 was, I believe, the last to use coils with that bad insulation material between windings. None of the other 1937 models used that type of coil - Philco had switched to much smaller coils in most (if not all) other 1937 models.

Before that, the 38, 60, and 89 in the 1936 season were among the last to have coils constructed in this manner, as the large coils used in the 600 series of models, and the 116, are made differently and do not appear to have any nitrocelluloid insulation.