First came 16704-56 (front) and 16706-56 (rear) and both these numbers are under the D-ring ledge. According to Bruce Palmer’s 37–64 second edition these two casting numbers appeared in late-56 production and they continued through the end of 1957 production. Palmer says when these heads debuted that the date code (DC) was dropped. However, while it is true that some of these heads do not have DCs, some certainly do so they may help you decide which ones may have been used for an early-57 model.
The other two casting numbers, 16700-56 (front) and 16701-56 (rear), are on top of the head. In his first edition, Palmer said heads with these casting numbers were first used for 1956 models but it seems he did not know then that 16704-56 and 16706-56 existed and he didn’t include them in his 1994 publication. In his second edition he says heads with 16700-56 and 16701-56 debuted as of 1958 models although they are not all exactly the same for 58–65. Anyway, 1958–65 heads have wider cooling fins than the previous ones. He also says they have DCs but again he is only partly correct because while some have DCs, some certainly do not.
Something else Palmer says in his second edition is that as of 58 model heads the number of fins on top of the head changed from six to nine. But he’s wrong because the change occurred prior to 58 models and you’ll find nine fins on top of some, not all, heads that have the first set of -56 casting numbers. In fact you’ll even find nine fins on top of some, not all, heads that have -55 casting numbers.
For Panhead front heads, below are the DC examples I have so far, beginning with 1947 castings:
J 7, K 7, L 7
A 8, B 8, C 8, D 8, E8, F 8, H 8, J8, K8, L8
A9, B9, E9, F9, G9, K9, L9, M9
3 0, 5 0, 7 0, 80, 90, 110, 120
1-1, 2-1, 5-1
5 2, 7 2, 8 2, 11 2
2 3, 5 3, 7 3, 9-3, 11-3, 12-3
2-4, 5-4, 7 4, 8 4, 10 4, 12 4
1 5, 4 5, 5 5, 9 5, 10 5
6 6
7-7, 10-7
5 58, 7 58, 8 58, 11 58
1 59, 3 59, 8 59, 10 9
1 60, 3 60, 8-60, 9-60, 10 60
1-61
2 62
1 63, 5 63, 6 63, 7 63, 8 63
Panhead rear heads:
J 7, K 7
A 8, C 8, E 8, H8, I8, J8, K8, L8
B 9, C 9, E 9, F 9, G 9, L 9, 12 9
1 0, 2 0, 4 0, 5 0, 6 0, 7 0, 100, 120
2-1, 4-1
5 2, 7 2, 9 2, 112, 12 2
3 3, 4 3, 7 3, 11-3
1-4, 5-4, 8 4, 11 4
1 5, 5 5, 10 5
1 6, 6 6
5 58, 7 58, 8 58
2 59, 3 59, 4 59
3 60, 6 60, 10 60
1-61, 4 61
1 62, 4 62, 10 62
2 63, 3 63, 6 63, 8 63, 10 63, 11 63
6 64
Notice a front DC of M9 for Dec 49 and this suggests the letter I was skipped. For a while it seemed that I was indeed omitted from DCs on all BT aluminium parts cast in 1948–49 because some pieces showed up with codes such as M8 and M9, including some BT trans lids with M8. However, later on some other BT trans lids that appeared to have been cast in 1948 showed up with code I8. And you’ll see I8 in my list of rear DCs. Investigation continues.
Notice a rear DC of 12 9 for Dec 49 and this is the earliest example I have of numbers-only DCs on Panhead heads.
Notice for 1959 that most months have 59 for the year but for October 1959 we see 10 9. This change in format for some months in 1959 is also found on some other Panhead aluminium parts such as crankcases and trans cases but so far I only see it for October (10 9) and November (11 9).
I don’t know what E stands for but I’ve seen it many times on Panhead heads. Other lone cast-in letters I’ve seen are D and I but their meaning too is unknown to me.