If it's the porch light with the orange lens I'd bet on the 'ole 921 bulb. Do you have fixtures inside that look pretty much the same? Maybe one of those bulbs will have a number. Have you checked the owner's manual?
2013 F-350 CC SB 2WD 6.7PS
2013 Eagle Premier 351 RLTS
-SOLD- 2012 X23B
-SOLD- 2003 Ford Expedition 5.4, Bilstein shocks