My 531 Raleigh conversion has mudguards mounted with p-clips: 16mm diameter clips on front, 10mm on rear. Same guards were once mounted to a Globe Roll 2 frame in the past, but I used 12mm for those seatstays.
Could play it safe and go 12mm, and bulk it out with electrical tape, but you'll never know without measuring with a vernier caliper.
BTW can go smaller than 16mm on the fork (i.e. closer to dropout).
I measured/placed them higher up the fork leg because I wanted the mudguard stay to be horizontal.
My 531 Raleigh conversion has mudguards mounted with p-clips: 16mm diameter clips on front, 10mm on rear. Same guards were once mounted to a Globe Roll 2 frame in the past, but I used 12mm for those seatstays.
Could play it safe and go 12mm, and bulk it out with electrical tape, but you'll never know without measuring with a vernier caliper.