Nuasea ...
So in addition to Dora-9's info, I'm reasonably certain that your missing full serials are ...
WK635 - N546PU USA : Now back in the UK @ Goodwood (RN colours)
Image Credit : CoffmanStarter September 2013 @ Goodwood
WP781 - N31105 USA : Destroyed
WG363 - G-BDIM : Destroyed
WP 778 - G-BBNF : Destroyed
WK 609 - G-BXDN : Believed to be still airworthy/flying (Google for pic)
WK 548 - G-BBMZ : Believed to be still airworthy/flying (Google for pic)
Source Reference : Book : DHC-1 Chipmunk "The Poor Man's Spitfire" ... with due acknowledgement to Dora-9's contribution to that book