Hi ....
I've never seen a Savage with a butt disc, but there's a lot smarter folks about Enfields than me here.
As you mentioned, but I'll repeat it here for collectors who may also be trying to assess the correctness of a Savage against an original issued one, go to the England - Milsurp Knowledge Library (click here) and view the 82 picture photo gallery montage for the 1943 Savage No.4 Mk1* Enfield Rifle (click here).
Also on the subject of Savages, with thanks to Advisory Panelmember Lance, there's also a rare "all correct" 1942 No.4 Mk1*(T) Savage Sniper Rifle (less scope) (click here) .....
Lance's article, accompanied by a detailed 165 pic photo montage, speaks to this example as being one of approximately 3,400 Savage rifles converted by Holland and Holland and re-barreled with an Enfield 5 groove barrel, however, it was never fitted with a matching No.32 scope.
Hope this helps...
Regards,
Badger