What does the D mean?

[TactAdv]

I'll stick around Peter...I only had one Bren but it was a Mk1...so long ago I can hardly remember.

Well, Peter, not everyone is afflicted with the disease to the same degree.

I do also note too that until this 'D' question popped up here......said forum was in a near month long state of silence. Primed the pump so to speak.....

Now, I do have a serious question about these Daimler-made guns (receivers); who here besides me has a still-functional machine gun with a Daimler-made receiver? I'm asking because it relates to something I am curious about that only matters to still original, uncut, receivers. Anyone who has one too I'd like to talk with you.
-TomH