Mystery Type Font

[John B Easson]

I have just been given this odd font of type - the question is, just what was it for? No, I've not done anything to the photo, it's really like that.
The only clues are those odd extra characters: a number of large plus signs, and some small zeros (or Os) in superior and inferior positions. I'm also curious as to who made them - there's no pinmark, and I've never seen such a font listed. It's type-high, and 18 point. Presumably it was for use in connection with some trade or industry, but if so, which?
(I assume printers will of course notice it's reverse-reading when printed! My workshop sign is 'Press Here' but reverse-reading, so I'm going to be using it for that.)
UPDATE: Thanks for suggestions - by chance I had a visitor today who has used such type! Apparently there were only three sizes of Franklin Gothic made this way, and were used on a Rejafix hand platen to print onto a rubber pad. Scientific glassware items were then rolled on the pad to pick up the ink (presumably a special one), which was then fired to harden it onto the glass. Blocks were used for measurement scales, etc. There may of course have been other uses.

[Dave Hughes]

I tried getting a clue from the characters that had been inked!

A C E O and a few numbers. Looks like a couple of the hyphens have been used as well.

[AlanB]

Right reading type is used on offset letterpress machines like the Rejafix. Or you could print on the back of a translucent substrate.