The Instrument Unit was retained after Skylab reached orbit, and was instrumental (sorry) in configuring the station for orbital operations.
From Skylab: A Guidebook:
Control of the Saturn V launch vehicle during launch and powered flight will be accomplished by guidance and control systems located in the Instrument Unit. This function will be maintained by ...
With some googling I found a web site with a reference to the F-1 test stand being at the coordinates 34.944, -117.688. Following that to the Google maps link, in satellite mode one clearly sees the remains of 2 test stands. The one on the right matches the F-1.
As for Santa Susana, an article in the Daily News has a picture indicating there was something ...
During static test firing, the kerosene-based RP-1 fuel left
hydrocarbon deposits and vapors in the engine post test firing. These
had to be removed from the engine to avoid problems during engine
handling and future firing, and the solvent trichloroethylene (TCE)
was used to clean the engine's fuel system immediately before and
after each ...
A "throat plug" is a cover to prevent contamination from entering the engine's combustion chamber from the direction of the nozzle. It can also serve as a "stopper" for leak testing.
I could not find a picture of an F-1 throat plug, but here are some diagrams of the Space Shuttle Main Engine's throat plugs.
This diagram shows the upper ...
Looking at for instance the list of events for Apollo 11, there are no "staging events", just a number of "separations".
On the other hand, if we are counting everything with a "stage" in the name, there are a total of five stages, three on the Saturn V and two on the lunar module.
To make a slightly different point than Russell ...
Warning: this answer contains unsupported opinions.
I don't believe the events after dropping the S-II are generally referred to as staging, but I think at least the S-IVB/CSM separation event could be considered staging.
Separation of S-IVB on one hand, the LM+CSM on the other hand. Not clear if this is considered as a staging event.
While not usually ...