Need Help Identifying Cancel On Postage Due

I may have posted this in the past (years ago), but I can't remember.
Anyway, can anyone identify the cancellation on this postage due stamp? I don't really collect US, but it's been bugging me for a long time.
Anyone?

Doesn't look like a Circle F....maybe a Circle R? Railway PO cancel?

I would agree it's probably a railway killer cancel.
Looking in the book U.S. Route and Station Agent Postmarks (C.L. Towle, Mobile Post Office Society, 1986) there are a lot of agents cancels shown using large letter obliterators. Some are in circles like, yours, but others are negative in black circles, or free floating letters. Most of those letters are N S E or W, which I think pretty clearly denotes north south east or west. There is one large B in a circle, as well as a V and an L, but I have no idea where they are going with those.
I'm going to assume yours is a mis-inked E.
It could be any one of several railroads that use the letters in a circle, there were three or four. If we can assume it is directional, then we can also assume they all use the letter E at some point.

I would politely disagree. U.S. due stamps tend to be canceled in the place of final destination as the due amount is about to be collected. A railroad mark (typically applied at the origin or in mid-transit) would be very unusual for such. I suspect a different explanation is correct, and linkable to a specific town, but I do not know which one.

Looks similar to Lf-10
Greeley Colorado
1875
Fancy Cancel. (F in circle)

Interesting guesses so far. Trying to google as much as I can along all the lines mentioned but so far haven't found a similar example. I even tried a Google reverse image search.
It does look it could be another letter besides an F, like B, R, P or E.

Rod, is there an online reference for fancy cancels?

Not that I am aware of "Spain 1850"

I would bet your wallet that it is an "F"
Greeley Colorado has a lump coming down from the top arm of the "F"