FWIW the ammeter part number doesn't show in the CMP parts lists I checked (C15A-04, C60X-04, C15TA-02 and Illustrated Army Parts Catalogue No. 175).
The part number also is not listed in the Chevrolet Master Parts Catalog 1929-1948 edition (includes both car and truck parts).
Is it possible that the ammeter was made by Delco, in the same style as used for Chevs, but for use in another manufacturer's vehicles? (except for the eBay seller's description).
Another eBay seller has the same part to model match
http://www.ebay.com/itm/NOS-1940-Che...-/221223477763
More web searching has Jimmy Carter and Tim Tygart with the same part to model match, so maybe the Master Parts List made an omission?
Sometimes, the more I look, the more confused I get.