HELP! I think I've found an original 604 in a 605 Cabinet..

[Alien_Shore]

It will be necessary to pull the speaker out of the cabinet and inspect it closely if you intend to put it up for auction and get top-dollar for it. Collectors will want to see high-quality close-ups of the speaker and cabinet to know if they are bidding on an original. Here is a good example of some of the details that you'll want to look for in an original "RWB 604"

Owned Played by Les Paul Altec 604 Hollywood RWB Speaker All Original | eBay

You will want to check the functionality of the speaker as well. Using your DVM, set it for ohms (DC Resistance) and check both sets of speaker terminals. You'll want to take pictures of those readings as well to show the voice coils are in-tact. As you mentioned, you can hook the speaker up for a listening test, although the crossover is likely to be in pretty terrible shape; anything around since WWII probably has some "issues" Let us know if you want help in how best to hook it up and conduct a listening test.

As far as the cabinets go, someone already mentioned using Howards on them - look at your local box store for Feed-N-Wax. It's great to revitalize a beautiful old hardwood cabinet like that.

Also, for fun, here is one of the original marketing pamphlets of this speaker from the mid-40's:

Altec 604 Introduction Brochure