Cal. 320 vs. 321

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Hello!
I found a very attractive watch online - it is a 2-register De Ville - but to my surprise it is coupled with caliber 321, which, under my understanding, should be associated with 3-register dials (and I would expect cal. 320 in this watch). Is it safe to assume that this is a frankenwatch or are there any exceptions?

Thank you!
 
Posts
33,527
Likes
38,196
Hello!
I found a very attractive watch online - it is a 2-register De Ville - but to my surprise it is coupled with caliber 321, which, under my understanding, should be associated with 3-register dials (and I would expect cal. 320 in this watch). Is it safe to assume that this is a frankenwatch or are there any exceptions?

Thank you!
It’ll be a frankenwatch, 321 is three register 320 is two.