It does look wicked sturdy for a marker.
If it was for a fence or gate you would likely
see mounts on it or rusted studs, straps,rust marks etc.
Silly question, why couldn't that be a 58 ??
Guys,
This might seem odd, but that almost looks like some sort of utility marker,
which would probably support it's date more in the 1900's.
Would you consider it too far off any trails,roads, etc to be relevant to any utilities past or present ??