I am not overly familiar with the 17 pdr brake system, but here is a shot of the 105 howitzer Warner electric brake system. Drum size is 14"x2". The 105 got rid of the electric brake portion of the setup but retained the handbrakes.
Is the 105 similar to the 17pdr? If so I managed to find a set of brake bands on ebay a few weeks ago.
http://img.photobucket.com/albums/v2...etruck0013.jpg