Sorry, no offense intended!! I was referring to those who (present company excepted, of course), after making a flat and absolute statement, conclude by stating or implying that this is the only correct answer and permit no exceptions.
The subject is very complex, and the complete history is (IMHO) not yet known and very long in the telling.
I'll give a generalization, which I believe is helpful. There are 3 general groups of early annual (prefix, year of sale) numbers. 1930?-1959: the 6 & 9 have straight backs and round closed ends, although the exact style may have varied 1960-62: the 6 has a curved back and round closed end 1963-68: resumes the pre-1960 style, the 6 & 9 have straight backs and round closed ends, although the exact style may have varied 1969: the 6 & 9 resemble "music notes", the tails are straight but the closed end is not round but elliptical with the long axis horizontal
Thanks for the link, Chris. If I ever walk up to a group of people 'discussing' numbers and spot a liquor bottle, I'm leaving... I was pretty sure my 41 ULH was good to go, then I heard this open 4 stuff. I was glad to see the closed 4 on the 40 - 42 models. I guess there can be an exception that too. If you're around long enough you probably seen a lot that's not documented.
I'm so submerged in work (Power Plant) these days, that it may be next year before I'm able to get in quality shop time. Other than the few hours (maybe) a week that I'm doing now. Anyway, I had to tear down the engine and transmission on my '59 FLH, due to the engine seizing, about a month ago. Good long service, since I bought it in '74. I knew it was tired when I bought it, and never ran it for extended periods over 45 - 55 MPH. So, it was saved because of this practice. I ride it for the 41 mpg that it gets, rather than the 15 - 17 that the truck gets.
Anyway, my point is that my engine numbers have all the appearances of being original. No boogering of the boss. But, my year number has a straight-backed 9. And, the sequence/serial number has a curved-back 6. My thinking all along has been that the 9 would be flipped over to make the 6. The production number has a straight-backed 9 for the year, but no other 9 or 6 in it.
#1 ("37WL2") shows a 1943 (probably WLC military) bottom number.
#2 ("37WL2a" is this another view of the same case?) has a curved 9, and violates a minor cosmetic rule "the numbers may not be on the same horizontal line, but will generally be spaced at equal distance from both ends, and have similar spacing between the stamps."
#11 I'm not comfortable with this, because the 2 letter stamps are different types: plain (like Verdana, Geneva, Arial, Helvetica) for the "E" and serif (like Times, New Roman, etc.) for the "L". The boss appears to have been re-shaped, and there is potential evidence of welding in the boss (pinhole to the left of the 6), and on the case slightly to the rear of the boss.
#33 looks weird, but not because of the numbers - because the boss looks altered, but that may be harmless.
#37 may be real, as an extension of the "curved is OK for the 1960-62 year" rule, but the texture is far too busy to suit me.
#41 the 6 used in the year is obviously not the same stamp used for the 9 in the sequence, it's a straight back (not common 1960-62), and appears to have been done with 2 separate strikes or a badly fabricated stamp - note how the circle enters the back at a 90° angle?
I HAVE TO SAY THIS IS A GREAT POST TO IDENTIFY IMPOSTERS BUT IT ALSO GIVES THE BAD GUYS ALL THE INFO THEY NEED TO MAKE UP STAMP SETS THAT ARE CLOSE ENOUGH THAT MOST WOULD NOT KNOW THEY WERE FAKE
IF REAL #S ARE THIS HARD TO CATOGRIZE HOW HARD WILL IT BE TO TELL THE FAKE ONES FROM THE REAL ONES 20 YRS FROM NOW?
I JUST LAST WEEK HAD A FRIEND ASK ME IF HE SHOULD BUY ONE OF THOSE FAKE # SETS OFF EBAY TO REPAIR A DAMAGED REAL # HE HAD NO ILL INTENTIONS AT ALL
I TOLD HIM NO WAY
I KNOW ANOTHER GUY THAT DID BUY A SET AND MATCHED THEM TO ORIGINAL #S ON CASES HE HAS
THE NEWS WAS THAT EVEN THOUGH THEY WERE VERY CLOSE NONE OF THE #S WERE EXACT IN STYLE FONT OR EVEN THE EXACT SIZE
IN CONCLUSION I JUST THINK ITS NOT GOOD TO GIVE THESE CROOKS ANY MORE INFO THEN THEY ALREADY HAVE PLEASE THINK ABOUT WHAT YOUR DOING BY POSTING ON THE INTERNET AND LETS KEEP THIS INFO AS FAR AS PIX GO BETWEEN TRUSTED MEMBERS AND NOT OUT THERE FOR EVERYONE TO SEE