Serial number that doesn't seem to fit the formula

[audiomagnate]

I have a 1990 model year SR 400 3.0 Criterium frame with this serial number stamped into the chainstay: 65610890090
The seller said it was a 54cm but I think it's a 56 because it measures 22 inches or 55.88 cm from the center of the bottom bracket to the top of the top tube and 23.5 inches or 59.69 to the top of the seat tube, and there's that "56" in the serial number. Any guesses?

 

[Brian]

6 56 10890090

It looks like you have a 56cm. And either a 89 or 1990. In 1989/90 they had confusion. Just like 1999/2k.

your SN is either: 1-08-90

or 10-89

I am going for made in 1989. so either a 89 or 90...

or it could be made in 1990...

 

[audiomagnate]

6 56 1089 0090
After checking a number of other frames and measuring my rear dropout spacing, I'm confident that these 3.0 Criterium frames follow the rules for 1988 - 1992 version 2 frames, except that the frame measurement is in centimeters instead of inches. So the 6=126mm rear dropout spacing, 56=56cm frame size, 1089 is October 1989 manufacture date, and 0090 means it was the 90th made in the series.