Hi Barry, front and rear brake shoes are the same on the military M20 and are particular to the model, being wider than most BSA shoes..they are 7" diameter. So if you stipulate WD M20 when enquiring you should get the right thing.
Martin Bratby has them and Russell Motors still have original linings and rivet sets if you want to do them yourself.
There are two types of shoes..both are cast aluminium and dimensionally the same. However, Wartime shoes have a much higher Zinc content in the casting than earlier or later ones. They are easy to spot as they feel unaturally heavy for Aluminium.
There can be problems with these if they are supplied to a relining facility for the application of bonded linings. The bonding temeratures are high enough to make the material go 'plastic' leading to distortion of the shoe, the effect of which is to make the radius of the shoe different and they no longer fit the hub correctly..There are also Indian made replica shoes on the market..I haven't used these so can't comment on the quality and accuracy, but if you purchase new shoes check them carefully against your originals before fitting....Ian
email (option): ian@wright52.plus.com