[Ron Ramirez]

You're partially correct.

The dial and shadowmeter lamps are indeed type 46, 6.3V @ 250 mA.

The "cabinet lamp" (which illuminates the grille) is an appliance or sewing machine lamp rated at 120 volts. Notice how this lamp, part (65) on the schematic, connects across the primary of the power transformer so it does not illuminate unless the radio's power switch is "on". A 6.3 volt lamp's filament would immediately vaporize at 120 volts.

The original cabinet lamp was tubular and frosted. Replacement lamps are not frosted, but will not normally be seen unless you get down on the floor and look up at the top of the radio's grille.

I do not own a 15X or 15DX, but I did have a 15X once years ago, and remember replacing the bulb as part of the radio's restoration. I remember the bulb was tubular in shape and may have been a sewing machine bulb with a screw base. I do not remember what size the base was - whether it was candelabra, or that odd size in between candelabra and a typical lamp bulb. I still think in terms of "old school" and am not yet up to speed on the various "E" sizes of bulbs.