Skylab Cutaway Credit - NASA

McDonnell Douglas (Seal Beach, CA) did a study on modifying the to modify the Skylab as a moon-orbiting observatory and station

Jim Craig recalls:

I proposed this to MD in 1970 (as a 12 year-old!), with sketches and mission summary that I hand drew. The proposal would sent a "wet-tank" S-IVB stage Skylab to a moon orbit, the crew quarters to be further outfitted by subsequent Apollo crews after excess fuel purging. The station would conduct in-depth scientific surveys and mapping of the moon.

A company vice president sent me a nice letter in response, along with a copy of their own internal study of this mission (great minds think alike!). Unfortunately, I have lost both over the years.