ss carbs
 

600ss carbs- BDST 38 B 156
900ss carbs- BDST 38 B 67

Apart from the jets/needles etc, is there any difference. They most certainly look the same.

Carb manufacturers tend to use a number added to the end of the 'generic' part code to indicate the build spec of the jets, slide, needle. Its therefor not surprising to see that the 900SS number is 'earlier' than the 600 which was first produced much later.
If you fit a dynojet kit, most of the spec differences will be lost :)