Marty is exactly right.
The factory was interested in rolling complete machines off the assembly line and the color of a horn body or the smooth vs webbed oil lines would not make a part unuseable or not meet spec for production use.
Neither of these distinctions are noted in any Honda literature, such as parts manual.
As far as what or when vin grey vs black body horns were fitted, there is no rhyme nor reason, at least as far as vin sequence, that i am aware of.
It is a matter of conjecture as to why the two different horn body colours were used. Honda did not manufacture these horns, Honda provided the specs for the horn to the manufacturer (Mitsuba) to meet. The specs Honda provided most likely did not include body color. The specs would have been for voltage, decible minimum and fitment requirements and general appearance, i.e., plain horn v.s. chrome cover.
As long as the part met the spec for production use, a horn was a horn was a horn as far as having complete machines ready to roll off the assembly line into dealers hands.
I would go with Marty's observation, given both his 1849 and 2435 vins have black body horns.
Let's keep updating this thread with more info !
Who has the latest LH horn mount frame which came with a grey body horn
?