I was in e-mail contact with them, and they were very friendly and helpful. Yes, this is a conversion they did for a customer using a HF M4A1. They did not tell me how much this conversion would cost, but they made clear how hard it would get to find a HF M4A1 in the first place. Believe me, I know what they are talking about. The problem is that there is no old stock, and numbers were very limited, although HF ran two production runs. Even when it was available at some shops this model went for 2200 to 2500 USD then. And collectors wanting one bad enough to pay that price are in general not wanting or in need to sell it again.
Anyway I like the HK 416, but there would be no way in hell I would allow anyone to modify my HF Colt M4A1 if I happen to get my hands on a very good piece.