Boa,
Having owned a 2 1/2" Colt Diamondback I know they made them only in .38 Special. Having said that, I don't doubt you saw what you describe, however.
Did you inspect the markings carefully?
- Markings are of the same depth
- The same "typeface" was used for 357 as for 38
- The spacing between the calibers looks even
- The numerals 3 5 and 7 are lined up (upper & lower) with 38.
- The spacing between 3, 5 and 7 look correct compared to 38.
- Does the word "Magnum" appear on the barrel or frame?
If "no" to any of the above, I'd be highly suspicious. I'd insist on getting it lettered by Colt as was suggested.
The BIG question is -- will it accept a .357 Magnum round in the chambers?