I have some early T48 magazines, but they are Belgian-made, not US-made.
The early ones are metric, with silver / bare metal colored followers.
I don't think it is hard chrome, but maybe or maybe not stainless steel.
I think the final, US-made T48 magazines are inch pattern, as the Canadian blueprints were used for the final versions, made in the US.
Before H&R, the early versions of the T48 were made in Belgium and the earliest were metric.
The Americans requested the long flash hider, due to the ball powder the US first used and right hand threads, so nobody got confused and damaged tleft hand threads.
The British weren't happy with the metric magazine and requested the inch magazine, with a larger front tab.
For the original inch pattern, besides being able to share parts, each country was supposed to design their own set of blueprints.
The US trials were too soon to get that done, so the already completed Canadian blueprints were used for US-made T48's.
Australia figured the expense of an even smaller country designing their own blueprints wasn't worth it and also used the Canadian blueprints.
The British wanted to tweak the blueprints a bit longer, to better suit their manufacturing and to beef up a few more weak areas and so the British blueprints were the last and ended up with a slightly different looking receiver.