Now...the pn is assigned by the first use, not only use. Ford trucks and commercials bear many parts with passenger PN and vice versa.
The 81 was first used in the USA as an econo accessory for the '36 commercials used in slow speed service, and was not recommended for any other purpose until 1937 when we got the 60. I think it may have had a dif PN before the 60 went into US production...
the 97 was ALSO first an over-the-counter carb for economy in 1936, then it rapidly took over as a full production carb. It was ALSO first recommended for city commercial use, hence the 67 instead of 68!
This stuff is somewhere in the '36 bulletins.
In general, prefixes are assigne by first vehicle the thing was planned for, and most remain unchanged until there is a design change.
Oddly, the conspicuous 81 and 97 labels have no reference to either Ford or Stromberg systems.
Use of just the prefix was common at Ford...they figured that the parts man knew that he was looking at a carb or whatever, and the prefix was sufficient to tell him which carb. That way he could quickly match it to the parts book and did not need the sort of detailed scrutiny it would otherwise take to tell one Stromberg from another.