T wrench is about 4 inches long, fits a 5-sided Polygon nut.

shows "MCD" and serial# 4510103 on other side.

Also shows "MFC" on round end

I flipped the pic so letters are upside down.

The only five sided bolt head I am aware of is on the top of fire hydrants, but there are probably more in industry.   Firemans wrench is usually 24 inches long at least to open the valve.

I'm like you John, the only modern 5 sided fastner is for fire hydrants, but back when some cars had 3 sided hubs so who knows.

thanks....
I was thinking a value wrench

Do you mean a VALVE wrench?

oops yes

The other place 5 sided nuts are/were used was on access covers for water meters so people wouldn't open them up and change the readings. Since that wrench doesn't look beefy enough to turn a hydrant I suspect is is a meter wrench....

The telephone company uses 5 sided bolts on manhole covers

I also have this wrench, although the only mark "might" be the letter W, but is hard to read even with magnifier, my kindergardener says it's a W tough.  This one has a little tab on the outside that sticks out about 1/4".  I also thought of fire hydrant, but checked it and it is way too small.  I never asked the previous owner, but he seemed to have a lot of plumbing items.

Pentagon wrenches are measured along the flat, that is, how long is one flat side.

Standard water meter box keys are 5/8 (#1) or 3/4 (#2)

Curb box cover nuts (manhole covers etc) are 5/8 or 7/8

Certain Buried meters also use a 27/32 pentagon nut for the lid....

Gang,

I recently acquired a modern chrome Snap-on 5-sided socket. It has a part number but no size.  I tried a search on the SO website, but I got a message saying this socket is no longer available.

If anyone is interested, I can probably find this socket and post a photo or two.  Also, I can take a measurement of one of the flats.