From U-Boat.net.. (U-441, U-256, U-621 and U-953) was converted from VIIC to Flaks. So U-673 was indeed a VIIC with bridge conversion.
see
http://www.angelfire.com/co4/u_673/ If just could remember where i saw an article where was told that some forward platforms converted to stand above legs like U-673 to improve
diving speed. U-441 type forward decks increased diving speed too much. I wish i could find more pics of U-441, U-256, U-621 and U-953.
It might be interesting conversion in 1/72 VIIC.
U-Flak and VIIC/41 CT comparison...
-Anakin-