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AFAIK the first109 models used roll pins to secure the adjustment knob.I think I've got an old 109 with that thinner recessed clip, and a second one where the clip was replaced with a piece of wire. I'll bet @axxel57 can help with the history on that feature.
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You are right, just found one of these 109er with a 5 in the beginning.Looking at a 109 with thin recessed style retaining clip, the S/N on this one starts with 5.
Part number for the early thin style retaining clip is 109-71, as show in attached diagram.
Once the "Standard" style retaining clip appears the part number is 11-109-121, all available diagrams show this P/N used on 109s and Balanced Adjustables (including the true 156 versions) until production ended.
Interesting find!I've been collecting photo samples of uncommon 156 Balanced Adjustables with P/N 11-250-103 exhaust valve (as opposed to the common 150 Balanced Adjustables with smaller 11-109-128 exhaust valve). So far I've found nine examples of the true 156, and out of the nine there are six variations.
Photos below showing a recent ebay listing of 156 without the CE stamp, so a correction to my earlier comment in this thread when I had assumed all 156 will have the CE stamp.
The 156 variations include style of adjustment knob, diaphragm, finish, and body. The only common denominator is a matte/satin finish body (cover can be satin or polished chrome) with 250-103 size exhaust valve, and so most likely the new style 01-043-211 exhaust tee is also used on all.
As a side note, it is interesting to see the 1995 CE 156 diagram with original version single o-ring s-wing balanced poppet. This is the earliest example of the s-wing poppet I can find, diagrams of G250 show the non replaceable seat poppet until 1997 and only switch to the s-wing poppet in 1998.
I have two 109s with six digit S/N, both with modern 11-109-121 retaining clip.If I understand it right in the beginning of 1974 SP changed their numeration for the 109 from six digits to seven digits.
The first 500000 numbers were reserved for the 1974 109ties.
Don't know why the numbers above half a million became either again roll pin models or already the ones with D-Clip, but from 1975 again the ones with the thin clip and slot.
I think I have a couple of the ones with the finer texture, they are usually the older ones.I have two 109s with six digit S/N, both with modern 11-109-121 retaining clip.
The S/N on these are 912952 & 260949.
Interesting to note that #260949 has a unique adjustment knob that I don't recall seeing before. At first glance it appears to be the common textured version in well worn condition, but closer inspection revels it to be an actual variation.
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