Advice needed replacing 45 year old spigot
As you mentioned, the fact that spigot has two flats in front of the flange (for a smooth jaw wrench such as a crescent wrench or monkey wrench (but not a pipe wrench (serrated jaws) which would squeeze the brass spigot, possibly deforming the threaded fitting on the pipe)) suggests it is has female threads.

It is likely that there is some play in the pipes.  Remove the two nails and it might pull out 1/2", affording a view behind the flange. 

However, even if it is threaded at the spigot, its risky putting a wrench only on the spigot.  Those threads might be very tight, whereas a sweated connection some where under the house could be weak.  Thus two wrenches are required, one on the spigot and one on the pipe side.

Alternatively, you could buy a small adapter to mount onto the spigot long-term with plumbers paste or teflon tape or whatever stops the leak, then attach the hose when needed to the spigot.  For example, one kind of adapter would be a quick-connect. Another type would be a splitter. Another type would be a swivel union. Another type would be an antisiphon coupler.