I think all you guys are right. Having worked at Ford and GM in the 1970s, I can tell you how a professional manufacturing organization build things. A truck got launched by a "broadcast sheet". These came out of a dozen teletype machines all over the plant. A vehicle only went out against a customer order or a dealer order. It was not "lets build a bunch of stuff and maybe someone will buy it." Many things you might think were just made in batches were ordered by that broadcast. We didn't make a bunch of different seats and put them in racks hoping to use them up. Seats were a "broadcast item". They got built to order, for a specific truck. There was a guy whose job it was to shuffle around the broadcasts so that we didn't have 10 air-conditioned trucks in a row. That upset the labor content and the guys on the line would get behind-- so the option-loaded trucks were spread out among the strippers.
You didn't want a bunch of money tied up in inventory. So it makes sense they built matched case sets but did not finish the engine. You made as big a batch of case sets as you trusted the machinery and supply. 1955 is before the interstates and it snows wherever they built Sportsters. But I am sure their intent was to use the cases in belly-number order. Coordinating changes is a big deal in an auto plant. So there is a record of what revisions are in which belly numbers. I suspect the engine was a broadcast item. If not teletypes, than a runner went through the plant dropping off sheets of paper that get stuck to the frame and the engine and the fenders since things are ordered in a specific color. Once again, I really doubt they painted up a variety of sheet metal and used it as needed. They would have to figure out what colors were sold out and then order paint and ..... no, its a broadcast item, so the guys in the front office can order the paint as needed and they don't have a bunch of finished inventory costing them money. The frame would travel down the line, the workers would look at that broadcast sheet, and make sure the engine broadcast sheet and fender broadcast sheet and options they put on all matched the frame broadcast sheet for that specific vehicle.
I think we built trucks to the VIN number by the '70s, but it makes sense Harley stamped the VIN at the end, maybe even after they started the engine. It would explain why the VINS are so sloppy. But it is also likely they got the VIN off the broadcast sheet, since serial numbers were just that-- before they became the legal thing called VINs.
So even outside of Detroit, I am sure bikes were built to order, not put together from batches of stuff laying around. Now that being said, this was the theory. I am sure the practice was much more sloppy. Dave pointed out Harley was in financial difficulty in the 50s, so they would rework triple trees and frames and all the stuff Jerry has discovered, I think he said 47 frame variations the first few years of the K-model.
So the tachometer option was surely a broadcast item-- you didn't want to order 100 expensive tachometers and have them sitting around. But the Z-bracket and the notched eyebrow were made by Harley, so there-- well yeah, you can see them bending up 100 Z-brackets and machining 20 eyebrows. That would account for the uniformity in the notch from the factory. They must have made a jig that held the eyebrow and there is a drawing somewhere that shows how to machine it. But remember, when they used those 20, they had to keep track of it somehow, so the guys in the front office knew to order up another batch of 20 before they ran out.
All this theory is nice, until you're "lines down" due to a shortage. By the 1950s labor and management were hostile enough that it is inconceivable that the management could tell the workers to "just keep track of things" and build as necessary. Passive-aggressive was a great Union technique and they would shout "not my job, TELL me what to do and when." So I am sure there were times they were out of tachs or eyebrows or Z-brackets, and those bikes got pushed to Repair or gosh-forbid Heavy Repair while management ordered up the tach or the Z-bracket or the "make from" eyebrow drawing. So this accounts for all the goofiness that we see in builds, including, "put last year's stuff on" both to use it up and maybe because the new stuff was stuck on a truck in a blizzard.
To me I am astonished Harley would add that boss to the eyebrow and not bump the casting rev, since the mold had to change. Its equally befuddling that they didn't make a separate part number for the notched eyebrow. The Union was not the only part of the business subject to laziness. Hence there are both the factory-machined eyebrows, and I suspect there are eyebrows notched by dealers or customers who installed the tach kit. So expect to see a lot of identical factory-notched eyebrows, along with a couple hand-done ones we just have not seen yet.
I assume you all know about
the engineer at GM that didn't bump the part number of the ignition switch when he changed it, since he knew it would get him in trouble that he even accepted the design in the first place.
So yeah, you are all right. K-models and Sportsters are both carefully crafted and totally half-assed and often on the very same bike.