Maybe a sample game that was prepared for export design ?

No serial number, no credit wheel and the multiple high score lights in the backglass.
Apron is different too.

I'd guess what ever it is, it was designed or meant for export. The high scores in the glass seem to lean toward an export. No serial number and not matching up with anything else seems like sample.

Just guessing - but as it doesn't match any of the 3 known production versions of the game - I'd say it's possibly a sample game all things considered. But again just a guess...

It's Not a production Sure Shot backglass - it isn't correct if you look closely. Sure Shot does not have those high score numbers and if it would have been it's in the wrong position at that.... (I own a nice Sure Shot) also the further information about the backglass not being a glass but rather plexi leads me to further believe that this is a pre production sample, test, mock up or such with an eye toward export.

I'd guess it's a one off machine that was used in development of making the sister export game for Sure Shot.

I'd lean toward what Darcy says... this made it's way into some employees hands at some point.

I think it's a unique game that was probably supposed to go to the trash or scavenged for parts at the factory when actual production was finalized. Somebody saved it. It may have passed through various private hands before you got it. Can't see the playfield arch in the photos but if there is very little ball tracking up top at the arch I'd think that would also lend toward this theory. The cabinet could have gotten its age and wear and tear through life as a player, stored, played, mis-stored and passed and traveled through various hands during the years.

Again my guesses and speculation but I think it's logical.