Providing your location wouldn't hurt; not everything is available through the Internet.

Sigourney643 wrote:

Any recommendations as to where I could purchase the back panel (cover) for my Underwood SX 150

This one you'll have to keep your eyes peeled for.  Underwood 150 through the very early Touch-Master series were very prone to losing their backsides--especially the very early SX-150s.  I've even had to paint them different colors because I couldn't find one to completely match the particular machine I serviced.  Anyway, you may have to be prepared to buy a junker or two, or if you scout the flea markets (swap meets for some) or secondhand establishments, or antique stores, you might be in luck.  The internet might be a source.  You're buying a pig in a poke when you buy online (I stay away from it altogether).  And do not buy one if you can't see that it has a rear cover panel.  You may be just buying another machine with an exposed rear.  I've been seeing those for the last thirty-five years--about as long as I have been in typewriters.But they're phenomenal machines--if you tune them up, oil them, put in a new ribbon, and let fingers fly!!