Lehigh Valley mileposts

Generally, LV mileposts were squares turned onto one point(I hesitate to say diamonds)with the mile cut into the the steel...but I was scanning some ONCT slides and found this shot of "A 365" in Farmington. In further research, I've found mileposts/markers with A, E and I. Does anybody know what these alpha-numeric mile markers implied? Thanks in advance

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The only thing I'm aware of is bridges were marked by signs with milepost and letter.

lvrr325 wrote:The only thing I'm aware of is bridges were marked by signs with milepost and letter.

I think that's the answer.

Where is the pic? Almost appears to be coming out of the Ryan spur, and IIRC there was a good sized culvert right near the turnout. And that would have been within the 365 mile.

The only issue with it is I thought that, say the first bridge within a mile would be 365 A, then 365 B, etc. I could see getting to E, but I?