I've got 5 more from a collector who wanted to help with the quest. This brings us to 22 out of 50 - not too shabby!!!
Sketch card 605, Page 7, last panel (A 605, B 7, C 12):
Sketch card 212, Page 13, first panel (A 212, B 13, C 43):
Sketch card 581, Page 11, first panel (A 581, B 11, C 31):
Sketch card 086, Page 8, second panel (A 086, B 8, C 14):
And sketch card 497, Page 8, first panel (A 497, B 8, C 13):
It would still be great to at least get to half - if anyone here knows of someone who might be able to help, please let me know!