I just checked the Moss site and they show 2 different types of shoes for the rear of the BN1. One for the early bevel gear cars, and a different one for the later Hypoid gear cars. Unfortunately, the early shoes are not available but the later ones are. All is not lost however as linings and rivets for the early style look like they are available.
http://www.mossmotors.com/Browse/PlateMenuProducts.aspx?WebCatalogID=2&PlateTypeID=1&SubcomponentID=20&ComponentID=4
Edit-
In case it helps, I looked up the rear brake shoe numbers in a Lucas catalog and it shoes the leading shoe as P/N GB41540 and the trailing as GB41542.
http://www.mossmotors.com/Browse/PlateMenuProducts.aspx?WebCatalogID=2&PlateTypeID=1&SubcomponentID=20&ComponentID=4
Edit-
In case it helps, I looked up the rear brake shoe numbers in a Lucas catalog and it shoes the leading shoe as P/N GB41540 and the trailing as GB41542.