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Your keyway number makes sense. One of my AXP's that I can easily get to now has a S/N of AC69535 and stamped 57-26 on the keyway.
Here is part of what Lud Sibley wrote about RCA CRT date codes. The number on our tubes seem to have an additional letter in the beginning and does not match his codes so it is hard to know how to translate it exactly.
Year letters were:
1944 H 1949 J 1953 T 1957 S 1961 N 1966 Z 1970 P
1945 A 1950 C 1954 X 1958 K 1963 F 1967 U 1971 Y
1947 N 1951 Z 1955 R 1959 H 1964 J 1968 T 1972 S
1948 F 1952 U 1956 Y 1960 A 1965 C 1969 X 1973 K
Note the ambiguous 15-year repeat of the letters, except for "P" and "R."
The month-number and year-letter sequence was reversed each month. For example,
"C6" designates Nov. 1950, whereas "6C" designates Dec. 1950.
FOR SPECIAL PURPOSE TUBES
High-value power and cathode-ray tubes carried serial numbers. The serial identification was a year letter plus a five digit number. The year letters for 1944-73 were the same as shown above for metal tubes, plus the following (reflecting the later demise of the Lancaster power-tube plant compared to Harrison).
1974 H 1977 N 1980 C
1975 V 1978 F 1981 Z
1976 A 1979 J 1982 U
The number part of the identification reverted to "1" at start of a new year, at least on
types like the 858B. On some types like 869B, the serial number substituted for a
date-of-manufacture code. (S. N. 27-1-1A, p. 3 [6-14-57 and 12-1-69]).
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Tim
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