Thin bulb caps made for #44 and #47 bulbs.
They will also fit on #555 bulbs but are slightly short leaving about 1/8 of an inch of the #555 bulb exposed.
Bally/Williams reference numbers:
03-8063-1 Blue
03-8063-2 Green
03-8063-4 Red
03-8063-6 Yellow
03-8063-12 Orange