The Bren M2 pistol is irrelevant to 922r compliance because it is a pistol, not a rifle. 922R only applies to "sporting rifle".
The Bren M2 16" rifle is 922r compliance, because it has enough US parts in it.
But if you want to turn a pistol into a SBR, it needs to be 922R compliant first before it can be registered.
You may ask WTH?
At one point, it is actually easier to ship a bunch of US made parts to Czech, and bascially make all the world wide CZ Bren M2 commercial items US compliant because the US is the biggest market.
Say I want to build 6000 units in one year, it could be way cheaper to contract 6 to 7 essentially parts in the US to beat the part count in the US, and then ship these parts to CZ republic instead of having people swapping parts in the US or finishing partially assembled items. Everyone in the world who are buying off the commercial lines will be the gun with US made parts and US compliant 922r. Now, you may have to play a little bit of game in Eu content and duty when these things move around in the EU commercial market, but the value of these parts are probably minor, it is the "count" that matters to the US.
Think of how SCAR and X95 coming into the US - depending on duty and trade deal, there are multiple ways of doing it. Even the US b&T APC 556 rifles.