Single tail lamp was stock. Passenger side tail lamp was an option. IHC used several different vendors, likely due to supply/cost, so the 'stock tail lamp' can vary a little.
My KB-2 was bone stock when I got it, the single tail lamp was a 6v bulb like this exact NOS one on eBay.:
https://www.ebay.com/itm/Vintage-NOS-NO ... 3581!US!-1Part numbers for lamp assembly:
K-D: B204
Cabco: 1209
Part numbers for the lens:
Do-Ray: P-1254 Tiger Ey