Hi, Don't take my word for it check this eBay seller a well know seller of Gold Star quality parts.http://www.ebay.co.uk/itm/BSA-A7-A1...orcycle_Parts_13&hash=item20ea35ef85
Now if you do a search for "baa clutch adaptor", you will find that many sellers sell the same item for both 'a' and 'b' series.!!
I fitted the A10 type (all that is on the market) to my mate's Gold star, measured the offset of the primary chain and them sent the adaptor to Phil Pearson who machined it for a small fee.
You may get away with the A10 type, as i have another GS by shimming out bearing the sleeve bearing (max 90 thou). This then may bring the crank nut into the outer casing, 2 ways to deal with this.
1. Fit an SRM crank nut and turn it down a bit and fit 2 gaskets on the primary covers and remove some material from the inside of the primary cover.
2. Make a spacer / disc to go between the crankcase and the inner chain case, effectively moving the outer chain case out.
have used both this methods
3. Instead of shimming the bearing sleeve, shim the front engine sprocket out only, you will need to get a proper hardened shim washed otherwise it will get hammered.
I hope this helps.