Hello, & welcome to the forum. You've certainly picked a choice rifle to start your Enfield addiction!
The scope & bracket you have are clearly mis-matched to your BSA 1945 rifle, as you intimate. The question of whether the scope & bracket have always been together is a little more difficult to be dogmatic about. It is possible that they have always been together, but it is most unusual, in my experience, to find scopes (where dated) mated to later dated rifles. In practice, all genuinely matched
British 4T's that I've seen (as far as I can remember) have been fitted with scopes made in the same year as the rifle, or made after the rifle. However, these are just my anecdotal observations, & there may be others that would dispute this. The scope & bracket could have been mated together as part of an ordnance replacement to a needy 4T, & the centre-punching of the screw slots could have taken place then. I can't be sure from your photo's if the very rough surface of your bracket is due to pitting that has been cleaned off (& the bracket does look rather as though it has been gently tickled at some point), or whether it is just a very rough casting. If the former, the cleaned up pitted surface would not be consistent with the very good exterior finish on the scope, supporting the possibility of them being mated together from different origins.
The 'D' prefix on the cradle clamps is just a letter prefix as commonly seen on Dalgleish produced brackets. They worked their way through the alphabet & just happened to have got to 'D' when your bracket was made.
Hope this helps a little.