Thanks for stopping by. It seems that the 335 would be the closest modern equivalent. I'm not sure what its original model number was. Looking at Billingham's site there have been a number of improvements to this design and there are also two larger models in the series now. It's an excellent bag and has stood the test of time. My bag has a very hard foam interior which I think I can probably replace with the latest materials if I contact them directly. We'll see how they respond!
This is the first of a three part case study. Parts two and three are completed and in the final stages of editing. They will be uploaded later this week.