I have seen mention of this book "Duocurve Bow" that Russell Willcox wrote & am aware of 2 manuscript-like copies in the UK.

Does anyone in the US know anything about it?
Was it ever published?

Anyway to whet your appetites I've scanned the "CHAPTERS"/contents page & at random within the "BOWS" found a nice page on the draw/let-off effect of his limbs - must be the Sid's favourite page (i'll have to ask them if they know anything about this book).

   

Any photos in that manuscript?

Not one! ;-)

Russ Willcox lived about 28 miles from me although I was not fortunate to have met him,too young :-) I do have a copy of manuscript (not pictures).I have had seberal of his bows and a good friend of mine has the last bow Russ built which was built for hi  another friend had Russ's personal bow.

What do you know about the manuscript?
How many copies? Who given to? Was it his intention to publish?
I'd be interested to see some pictures of your bows & your friends if possible.
I've seen a few pics of some of his bows in the UK
& there were a small no. of bowyers in the '60s that explicitly credited his influence:
Les Howis (Marksman), esp. the "Outlaw":
https://www.facebook.com/media/set/?set=a.915857615167715.1073741917.100002304553118&type=1&l=9c04657e8b  ;
Mike Hallam (Hallam Archery Co.) "Pegasus Duocurve") see:
https://www.facebook.com/media/set/?set=a.1147119298708211.1073741939.100002304553118&type=1&l=5f27308dbe