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Mine has no serial either...
My very unscientific steel test was tap it and listen to the sound and compare to my other steelies, it sounds very "thin" not like 531/631 so I concluded 853 (along with the formed tube shape which 853 only seem to have)
JWestland-
I like it...
I've culled my collection sufficient that I only have one steel left in the shed which is tange prestige so the tubular-bells method will have poor calibration, particularly since I've never been near a 631 frame in my life....
I'd used a similarly scientific, but slightly more pedestrian technique in which I squidged the downtube between thumb and forefinger, felt it deflect quite a bit and then thought "shit, that's thin.... must be 853!"
Another quick check might be to remove the BB and stick it on the scales and see whether the internet can tell me how much it should weigh...
Rik_Van_Looy
Brilliant, cheers - Also, the top tube cable routing & rear brake bridge are similar in style to your 559 CP.