being a hardware junky the xlch manifold bolt has been on my radar many decades because it has never been documented correctly in the parts books.
that said i strongly believe this bolt is 3/8-16 x 1-3/16.
hd lists part number for a bolt of that size: 4348.
(4354 is found in big twin parts books at 1-3/8 long. thats to long it will hit the lower carb screw head.)
as for the 4348 its the bolt that also fastens the rear motor mount to the crankcase.
there are two versions of this bolt early ones marked 1038cp and the later which are unmarked.
every non molested bike ive seen use the same bolt on motor mount and carb, with the 1038 used till at least e1960.
ive had my hand on a few good 64s and they always had the unmarked versions.
when the change from 1038 to unmarked needs more research.