If the price is troubling, and I have no doubt it is expensive, you could think about fitting the Merc copy bag and mounting block to your B.
It works - I've used it. This is my bike on a camping trip - we're talking about the front bag obviously. It has a quick connect / disconnect fitting and it just lifts off if you press the fitting.
Anita Clare who imported and sold the Merc copies, has since stopped selling them, but she had a lot of spare parts including mounting blocks and bags. As I recall the block used to be about a fiver and the bag was maybe about twenty five pounds (that's my recollection anyway.)
Does the Brommie come drilled and tapped for the fitting of the block? There could be a disparity on the threads with the merc blocks because the frame is aluminium and may have courser threads - but I'm guessing. Everything I know of is dimensionally exactly the same as the mid 1990s bromies which they were a copy of, so I doubt you'd have a problem fitting their block and bag if you wanted to.
She may or may not be able to help you. You can probably get her through her ebay Id which I am posting here. If not, I have her email address and will PM it if you contact me.
Last edited by EvilV; 02-20-09 at 11:04 AM.