Here's a blow up of 1958, you can see the months as Jan, etc. That's when they used the A, B, C, etc as the starting letter of the serial number, for Jan A, Feb B, etc. They skipped the capital I, and ended the year on Mnumber. That was becoming industry standard then. This easier to read chart has the month A, B, etc left off. It should be added in there, but by then it was industry standard if they had a year code as a suffix, the first letter would be the month. Keep in mind too that on the left side of the chart, the months are shown there, and each letter that started the serial numbers, like in 1948 etc, ran for about 3 months. The serials then went like D00001 to D9999 in that range, then changed to E and went again, etc. Later, using A for January, etc helped them to get more structure in the serial numbers. Whether it was 52Cw or H, etc. the year and month of build was becoming more exact and easier to keep track of. Hopefully, this makes sense to you. If not, I'll do better at explaining it to you.
View attachment 1437919