Could this be an unused 1903 mag?

Could be. It looks to be unused, but I don't know if it was made by Colt or not. Nice find though.

The magazine was originally blued. May have been a military refinish.

I'd be asking if it was a usable magazine, not is it unused. What does the latter matter, really?

There appears to be nothing wrong with the magazine as far as being usable. Colt magazines are no different than other Colt collectibles in that the better the condition the higher the price. An early Colt Model 1911 magazine in so so condition recently sold for almost $700 on ebay. The same type magazine in unused condition would probably sell for $2500.

I had planned to use it to replace the pinned mag that came with my type 3 1903. Not sure why it had an early model pinned mag but makes the deal I made for it even sweeter! I guess I'll keep the new mag unused, at least for now. And the search continues.