[WhippoorwillG]

Since the breeder hasn't answered, I will give it an educated guess.

Often, the numbers represent a specific line that has been selected from a breeding project. However, different breeders use different notations.

If it were my lines, it would represent seeds from a specific F3 selection called #1.

Saved seed from the F3 plant labeled #1 would be grown out (F4), and it would represent plant #4 in the F4 generation.

It may be none of that, but that is generally how I label segregating generations.