NEC Article 512 - Residential Storage Garages
Prohibited receptacles from being installed lower that 18" to the floor. The rule was in the NEC until the article was deleted in 1975.
That is probably a reference to what is now 511.3(B)(1) (commercial garages)
Of course if you wanted to buy explosion proof hardware I’m sure it would have been OK
Greg:
Here’s a copy of Article 512 from my 1968 NEC.
The question has come up recently, and although not in the code any longer I believe that if an inspection revealed a receptacle installed on the garage floor that it should be identified as a defect, or hazard whatever the mood brings. 
How was it in the code even then?
It is funny that I remember back in 1968, all garages I went in had their receptacles at 48" so they would not get hidden by boxes and junk and actually were more conveinient.
My own small garage here, the receptacle is at 48".
I’ve always took it for granted that it should be 48"
Marcel 
Just did a 1945 Georgian with Knob and tube.
First time seeing as an inspector.
48" makes sense to keep it above any workbenches or counters. That how mine have always been.