I wish I could help more, but I'm frankly stumped myself. I own six of the Stowe-era Briar Workshop pipes and I can't see much rhyme or reason to the numbers. It seems as if the better pipes have lower numbers.
1. I have a pipe marked "1" that has lovely, nice straight grain, but also a possible fill (I'm not sure); another one is marked "01" and has really nice flame grain and birdseye (no fills).
2. In contrast, I have a "5" that is stained dark, has so-so grain and a couple of obvious fills and a "4" that has no fills, but so-so grain.
3. Then there's one that has amazing, angel-hair straight grain, but is marked "3"
4. Finally, I have one pipe with nice grain, no fills and a very light stain (like Dunhill Root), but NO number whatsoever.
Whenever I get around to putting in an order of tobacco from Carole Burns, I'll have to just ask her. She was Nachwalter's wife and worked with the Briar Workshop and later with Nachwalter when he went out on his own with Pipeworks and Wilke.