the "D" designation sounds about right. Yeah, given the "approximated" history (I've short-listed below) I totally see it being a forth-generation of the ship with three (or more) main retail variants.
----Excalibur Revision History in Brief----
WC3 era
-prototype Excalibur (probably listed as "X-103" since "x" is typical for a prototype)
-several first-gen models ("F-103" maybe used in the Kilrah bombing run)
-eventual (post-war?) line models (2% to 5% less armored, less thrust, etc) (F-103A)
WC4 era
-few more line models (F-103B and assorted SR's (service releases) like F-103B-sr2 (~~~, like WinXP-sr2)...
notes: perhaps some weapon-swapping occurs, parts get updated, cost-cutting measures happen, etc
post-WC4 but pre-WCP
-development of the F-103C, F-103C-sr1, etc, more updating, most cost-cutting, etc
WCP era
-F-103D starts showing its age, gets more updates and/or cost-cutting,
notes: by now there are few original F-103A's left... the tech is less "cutting-edge-new" and more refined... its less tweaking for best-possible-specs and more mass-production line-models that have either seen combat or been tweaked for alternate specs (perhaps some are more tweaked to serve for roles as long-range missions, others are more tweaked for bombing runs, or close-range support, or whatever)
post-WCP
-the F-103D is still produced but is far from being "cutting-edge best of what the military has" and various SR's and PR's (Public Retail version) are around.
notes: quite possible that a "found and restored to near-mind condition F-103 with a few tweaked-in technology updates" would seriously kick a "line-model F-103D" 's tailpipe, given the toll 4 generations of cost-cutting and streamlining for mass production would have had.