The upper primary is a 1955 through 1957 rigid frame primary. The lower primary is a 1958 through 1963 swing arm frame primary. They are both foot shift since they lack the chain lube deflector on the upper rear. I've highlighted the three areas that changed. The nipple for the oil drain (along the lower edge) moved forward in the later years. The upper rear mount for the chain guard/oil tank changed (sorry it's hard to see the upper mount on the 55-57 primary, I did some repairs to it and it wasn't painted so tended to blend in with the background). On the 55-57 primary the upper and lower mount are the same. On the 58-64 primary the lower is the same as used earlier but the upper has changed. Finally, the location of the crankcase breather hole changed (moved down and to the rear on the later primaries). The 1964 inner primary has the breather hole in the old location, but the other two features the same, making it a one year only primary (sadly no photo available at this time).
The inner primaries on page 90 of Palmer's both appear to be 55 and later primaries misidentified as 40's primaries. I come to this conclusion based on the large sprocket shaft hole in the front of the primaries. The upper is probably a 55-57 rigid primary and the lower is clearly a 58 and later primary based on the location of the oil drain nipple and the upper rear mount (seen in the lower right of the photo).
Finally, in 1962 a rear chain oiler was added. This was a small dam bolted to the inner rear that collected oil and dribbled it onto the rear chain via a small tube through an hole in the inner primary. Sorry, no picture available at this time.
For comparison's sake, below is a shot of a beat up reproduction inner primary. Typically reproduction primaries
Finally, just for completeness, here is a shot of a pre 55 inner primary.
Notice the smaller centre hole at the front where it bolts to the engine
case. I've also circled the chain lube deflector as this is a good
shot of where it is positioned and what it looks like. Again, photo
is courtesy of an eBay seller.
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