Is there even a system? If there is it seems random! Take the 237 for instance which is a similar size to the 635/9. And then there's the question of fuel specific numbers for Coleman fuel/gas vs Kero/paraffin versions - I'm thinking of the 635/639, the 200/201 and the 335/339. Then there's odd numbering decisions like the 625 and the 635... totally different lanterns with similar numbers... and there's no 629 kero version... or is there?
I gave up on trying to make sense of it years ago. Likely someone has crunched all the numbers and found a pattern, but I am not that person!
