It is unfortunate that many parts stores no longer provide much help… NAPA is great that way for sure, but there are some others that can measure them as well if you press them to do it. However, the belt in question appears to be just a pretty standard cogged V-belt (4L), so there isn't really anything special about it. I have tried (not real hard though

to find a cross reference to a standard belt number, but haven't had much success. If you have the old belt, it can easily be measured or compared to a new belt to verify the size… shouldn't be very hard to find one.
Cheers,
Brad
PS: I do have a Delta belt and pulley size reference by part number, but it unfortunately is rather old, and doesn't include the belt in question.