These would be shaped like a standard concrete milepost but with have a letter beneatht he number. At present, no one knows exactly what these mean but it is presumed they indicate structures or what not along the track. Examples can be seen below in posts about this controversy.

Some say the letters indicate indicate culverts. Others contend that the letter G means cattleguard or gate. Would be curious to see the actual plans or maybe a book of SP specs for these sorts of mileposts. I call them mileposts because that's what they look like--but they may be some other kind of post like structure post or something.