need help identifing N frame cylinder markings

[AudieMurphy]

as Im going thru my excess parts Ive noticed on N frame cylinders various calibers..There are different letters stamped between the chambers. Does anyone know what these signify? and when did S&W start and stop this process. a few more are the capital letter "I" and "N" which I assume stands for nickle because it is nickle....Some have no letters at all...Thanks in advance

 

[DARE]

I only have three pre-war N frames. A 1st Model from 1916, a 2nd Model from 1926 and a .357 Magnum from 1935. None of mine have letters on the cylinders.

 

[s&wchad]

Not sure about the "F", but the "H" was stamped on Model 28's (Highway Patrolman) to let the polishing department know that it needed a matte finish.

I believe this was done in the '60's-70's.