The K-model "bullet" tail-lamp is part of our Hummer (and JD, VL etc.) Heritage. The 1920's through 1938 Harleys had a taillight that generally looked like those used on the K-models. The red and white lenses were similar, but the brackets were different. Starting in 1934, a "hood" covered the body of the tail-lamp, but that hood was part of the fender.
For the 1948 Model 125 (first of the Hummer-series), a new tail-lamp bracket was designed, and it included the license plate mount. "Harley Davidson 125" is embossed on the back of the license plate mount. A new run of the 68091-20 red lens was made, with -as Stan says- with "HD 125" in it. A removable "hood" was used, and it was painted black.
- 1948-1952 Model 125, 1953-1955 Model 165 tail-lamp
Lenses also used on 1952-1955 Model K and KH
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For the 1952 K, another new taillight bracket was designed, this time without the license plate mount. The clear and red lenses were the same used on the Hummers. The hood was the same as used on the Hummers, except that for the Ks, it was chrome.
There are small differences in the clear and red lenses used over the years.
Clear "license" lens1920/30's has smooth glass around the barrel..
1948-1955 Model 125/165 and 1952-1955 K-Model has "serrated" glass - ridges around the barrel.
Red lensI have to disagree with
Bruce Palmer page 576. Here's what he says (page 576):
There are also three versions of the red lens #5054-20 (new #68091-20). The 1920-1934 lenses have a slight dome to it. The 1935-1938 lenses, the 1941 WLA lens, and the 1948-1955 125/165 & K model lenses have rings inside as well as the beehive shape. The 1948-1952 125 lens has "HD" over "125" accross the tip of the lens. All the 1934-1955 lenses have the letters L.L. CO (Lancaster Lens Company), and the part number 2-60 on the inside edge.
Here's my opinion:
1920-1934 - Concur with Bruce, since I have no knowledge.
1935-1938/1941 - These have rings around the inside as well as the beehive shape, and are fairly dark red.
1948-1955 Model 125/165 - these have a pebbled finish (no rings) on the inside, and HD over 125 on the outside. They have L.L. CO 2760 on the inside edge. The red color varies - from medium red to dark red - believed to be just production variation.
1952-1955 Model K and KH - same as the 1948-1955 Model 125/165
My reckoning: Both the clear and red lenses were obsolete in 1939, and by 1947, Harley had probably exhausted their stock of replacement parts. So, for the 1948 Model 125, they had a new production of the red lenses made specifically for the Model 125 with "HD 125" in the lens. At the same time, they had a new run of the clear lens, this time with the ridges around the barrel. For the 1952 K model - they just used the Model 125 version of the red lens.
Spring RingsThere were two kinds of spring rings used to hold the lenses to the taillight body - both had the same part number 68105-20 (old 5056-20). The "early" or "type 1" one was a single round "wire" with two bent ends. The "late" or "type 2" one was a flat spiral ring.
Bruce Palmer says that 1953 and earlier were the "type 1" and 1954-1955 used "type 2".
Looking at the (notoriously inaccurate) illustrations in the Spare Parts Catalogs - the 1951 Model 125 catalog shows the type 1, the 1953 Model 165 catalog shows the type 2. The 1953 K catalog shows the type 2. Palmer may be correct for big twins and 45s, but I don't think this was a clean-cut change, at least for the Model 125/165 and the K/KH models. I suspect they simply "used up" the type 1's until they ran out.