I think it's quite possible that it was lettered by hand. Setting metal type on a curve, and reversing it, was practically impossible. Magnesium is a digital font with a similar style.

There were a million+ of these made. AC Spark plug used this font from the late 30's till early 50's on oil filters, air cleaners etc, and were screen printed on water slip decals.

I though it was simply Arial but the M and N have vertical legs, and the S E and B are different.

Are you saying that this might have been a font they drew up 'in house'?

Are there any sources for 30's and 40's era fonts?

I'm pretty familiar with what fonts were around in the '30s or '40s and I can't think of any that would match this. However, it was a very common lettering style, and lettering artists were a dime a dozen back then.

This is a typical example of the kind of thing that would have been lettered (drawn by hand) rather than set with type. Type (fonts) were not well suited for stuff like this. The lettering style would not usually match any particular font.

As I mentioned above, Magnesium is similar. There may be others, but that was the first one to come to mind.

You can search at MyFonts using '1930s' or '1940s' as the keyword. Remember, lots of pre-digital era typefaces didn't make it into digital form, but there have been plenty of them that have, and there have been revivals of some of the display types from that period, usually by new names.

- Mike Yanega

Also, there are quite a few fonts around now inspired by hand lettering from back then. A lot of what you'll find, following Mike's advice, will be fonts like that.

One other possibility is that AC hired a company like Photo-Lettering Inc. to set type on a curve using their photo-lettering machines. P-L did have some styles like this (I think--I don't have one of their catalogs handy), which were based on lettering styles. Not many of these have been digitized, though. (House Industries owns the rights to the P-L library.)