Friends,

The B&O converted older boxcars into coke gons by removing the roofs and tying the sides together with steel rods (that was the method in photos I have seen). This was done over several decades and the boxcars reclassified as various subclasses of O-39.This happened from the late 20s into the early 50s with different boxcar classes including some M-8, M-13, M-15, M-24, and M-26 boxcars. Confirming Elden's scenario, I saved a post from the B&O Yahoo Group from 2006 and it tells a similar story by first hand account and added it below.
 

Bruce D. Griffin
Ashland, MD
https://bomodeling.com/blog/

I well recall the coke cars in the early 1960s, probably the M-15 series.. The few I saw came to a local foundry. It lacked a between-rails receiving pit, thus the need for a car that could be unloaded similar to a conventional box car. Those cars I saw all had steel underframes and were very much showing their age with badly faded paint, wear, some rust and dents from probably loading.. When loaded, the doors had been cross boarded on the inside with planks. The foudrymen would climb onto the top of the car with flat shovels and start shoveling into an adjacent bin. When they reached a level even with the first board, the door was slid open and the plank removed. The men shoveled directly through the door into the bin. This was repeated until all the planks were removed and the car was finally empty. The planks were returned to the car loose, and the door then closed. They might put four to six men on the car and it would be easily unloaded in a day. I recall
seeing a number of those cars, along with the wagon-top cars sitting on a scrap line outside of a steel mill in Portsmouth, Ohio, just a year or so later. The foundry went over to trucks with the end of the coke cars and is still operating today even though the railroad tracks are gone.
One thing that caught my eye about the few cars I saw in service, they had the "Linking thirteen great states with the nation" logo, which was becoming rare in western Ohio by this time. The same foundry got coal in standard twin B & O hoppers, probably a 50 ton capacity, and they dumped it right onto the track and shoveled it off. All of those cars had a standard "B & O" painted on them. Those twin hoppers, all seemed to disappear about the same time, replaced by a 70 ton triple hopper. The B & O served a stone quarry at Piqua and every day they brought from 50 to 75 empty hoppers. The newer cars were too tall to fit under the tipple, so the quarry lowered the tracks, then a bunch of them lacked what the car knocker called a "stone gussett," which apparently was a corner reenforcement to handle the weight of the more dense stone. If it lacked it, the car was rejected by the railroad agent himself, and many were.

Scott Trostel