Jack, Indeed they are. The 1947 parts catalogue lists 67-2675 (o/s) and 67-2679 (n/s) exhaust pipes for the A7 and Star Twin Longstroke rigid frames. The same parts continue to 1949, Longstroke rigid and plunger frames, and are the same pipes used on the A7 Shortstroke, rigid and plunger.
So a 2 into 1 system from a post 1950 model should do. Experience shows they rarely fit from the box, and if possible it is better to get one which is unplated, then cut and shut to fit. The alternative is a used set, as at some stage some other fella had the hassle making it fit, but many are abused as getting the pipes to come apart usually requires a bit of violence.
Swarfy