<VV> No Corvair - Zenith Carb Question

[GYoungwolf at aol.com]

I am working on the restoration of several very old Zenith 28/228 series 
downdraft carbs (late 40's, early 50's) and discovered something I have not seen 
before, and am hoping there is someone in our group who might have. These carbs 
all have a steel insert in the accelerator pump cavity (looks just like the 
one on a Rochester HV) that essentially blocks it off from use. The carbs had a 
nonfunctioning (rusted and blocked) pump assembly installed in them. Any 
ideas as to the purpose of the insert? I'm thinking the carbs might have been for 
a special application like a generator or tractor that ran at a constant 
speed, but that's just a guess. I've searched my carb literature without success. 
Thanks for any insight and the space.
Grant Young