Zeiss ZM serial numbers?

[BradS]

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Regarding the modern, made in Japan, Zeiss ZM branded lenses for Leica M mount...

Some have serial numbers starting with 51 but most appear to have serial numbers starting with 15 - even for the same model lens (eg, 50mm f/2 Planar).

I thought I read some explanation for this but I cannot find it any more (and my memory is spotty). I tried asking the mfgr, Cosina but have not received an answer. 

Does any body know what the different prefixes signify?

TiA.

[Keith (M)]

No idea. All I can add (for what it is worth) is that both my 2010 35mm Biogon f2 & 2011 50mm C-Sonnar f1.5 start with 15...

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[Guest]

My 35mm distagon is a "51" but my 25mm biogon is a "15" .

My theory is 51 means "super duper duper" but 15 means its only "super duper" .

[BradS]

I have a 50mm planar and 28mm f/2.8 biogon that start with 15 and a 35mm f/2.8 biogon that starts with 51.  
 

I thought that maybe the 51 signified ‘originally sold in Japan’ or ‘after some manor change in production’ or something but...I do not know. 

Edited February 2, 2021 by BradS

[Theory033]

I purchased 2 new Zeiss ZM lenses over that past year, the 50mm f/2 Planar and the 35mm C Biogon f/2.8, which both begin with "51" serial numbers. As I have owned several ZM lenses in the past that always started with "15", I was curious about this too. 

While I do not know why, I have a couple of theories:

1. The latest lenses produced switched to the "51" serial numbers recently, as both of my lenses were purchased new in 2020 and I assume were made quite recently. However, there are no dates recorded on the papers included with the lens, the test certificate, etc. 

2. More likely, the "51" lenses are designated as Japanese domestic products intended to be sold to the Japanese market. I purchased my copies online through a reputable Japanese retailer and they were significantly less expensive than they sell for here in the US. I believe both the 50mm ZM lenses and the three 35mm lenses have domestic pricing that is somewhat cheaper than other markets (e.g., around US$600 each for the 50mm Planar and the 35mm C Biogon in Japan, compared to US$900 or so for either of these in the US).

I would be interested to know if there are any of the current 21mm, 25mm, and the 28mm lenses out in the world with serial numbers starting with "51". These seem to have the same pricing in Japan and the US, so there may not be a "domestic" version, hence, no different serial numbers.

Edited February 3, 2021 by Theory033