FWIW, the stock arm itself is made by Choate. In the early model, Choate supplied the entire unit, hence the bolt-on steel end plate with thick rubber pad. The newer models, the endplate is a factory HK MP5K plastic endplate and the Choate stock arm is attached to it.
Noteworthy that when purchasing a new one, consider the Choate at 1/2 the cost as you can replace the buffer if/when it wears out. The HK marked units you can not.