Eminent Technology ET-2 Air Filter

I’m the original owner of an ET-2, which has served me faithfully since the 90s.  I have the original ET branded pump, but the air filter cartridge has turned black with dust.  My filter is clear plastic with a white paper cartridge, and it’s about the size of my thumb.  Any suggestions on getting a replacement?
Motor Guard D-12-2 is what I use.  They're much more substantial than the factory filter as they remove moisture, too.  The thread is 1/4" npt, so you'll just need to get some barbs from a hardware store to adapt it to your tubing.
There's no reason at all to see inside the filter.  It won't do anything for you.  A loss of pressure will indicate that it's time to change the filter.  If you don't have a gauge after the filter, you really should think about getting one.  If you don't want one for some reason, the filters are cheap enough to replace annually or whenever depending on how often the pump runs.  Installing a gauge and only replacing filters when pressure drops is most likely the more cost effective option, though, since they only cost a couple bucks.
Being able to see inside is another level of personal trust that everything is  working well.

I'm in coming from the moisture side of things.
I see.  You could always look in the tube at the outlet of the filter for evidence of moisture, but I have never seen any moisture anywhere in my system.  I don't think it's an issue unless you're using high pressure.  Even if trace amounts of moisture built up in the filter, you should see a drop in pressure indicating that.

I plan on eventually connect my arm to my shop compressor.  When I do that, I'll be a little more concerned about moisture (and oil) and run a huge Motor Guard filter before the regulator and one of these smaller ones right before the arm for extra insurance.