Which lamp is this?

Time to replace the illumination in the radio and heater panel controls.

1994 Chevy Suburban. I've disassembled the heater control panel and found the lamps are a bayonet type. They are T1 1/4, and not available at the local auto parts stores (though not been to NAPA yet). There are LED replacements available on line but offered in 3mm, 4mm, and 5mm sizes.

My dilemma is what part of the lamp assembly is measured?

If anyone has a source for the lamp besides the dealership, I would appreciate know what the lamp is or where I can buy them.

Thanks all.