Values of mount in ID
bent_mast
straight_mast
pole
high_mast
wall
angled_mast
susbended
mast
lamppost
ground
cantilever_mast
90
ceiling
tbd

Then I found a link that doesn't list many either but the best match would be straight_mast.
https://wiki.openstreetmap.org/wiki/Key:lamp_mount
I find it very odd that so many choices are given but there is no documentation of what the things are. Is it possible one semi-colons different things together?  It is a lamppost after all. Anyway I'll deal with this tomorrow.  I have to get some sleep.