The McLean Museum sent me photocopies of their data. It looks like serial numbers for the 260 restarted in December 1960, with the introduction of the 260-A ending at serial number 101. The range stays the same through at least 260B. A flier for the Eureka range in October 1966 lists Eurekas 238, 240, and 260E

Data scans will be coming today.

The weird thing is that the serial number ranges are for 260, reset for 260-A, continuing with the B. My 260EK firmly falls in the range for 5/1964, but the column is for the B. Based on how quickly A turned into B (A was introduced with SN 101 in 12/1960, 260-B SN 17101, 10/1961), I'm thinking the bookkeeper just didn't update the letter, meaning C, and E could be in the data set for B.