As I understand things, Holland always made some of their own guns. In the early days, Harris Holland may have just sourced them from various people and put his name on them, or he may have "made" them, much like other makers made their guns.
For instance, Purdey the Elder never "made" a set of bbls or a set of locks. He made part of the gun (I think he was a stocker) and he bought in the other parts, had other work done, and oversaw the completion and finishing of the weapon. Then he put his name on it as the gun's maker.
This was standard practice in the trade and it wasn't until the makers started to grow and business really took off that they could bring all the work in house at a factory. Even then, they still farmed stuff out.
I think H&H built their factory in about 1890. At that time they started making their new Royal-grade hammerless shotguns and other guns.
But they still had stuff made by the trade. They continued this practice for a long time and may still do it today.
OWD
