The history of the Mark 1 reel's development is described in this extract from a September 1945 memo. It highlights the requirement that line used for controllable kites not stretch. Linen string used for aircraft repair turned out to be the answer. The ratcheting wheels from a push-type lawn mower provided the inspiration for another key feature of the Mark 1 reel, the line length adjustment mechanism.
Then there is an Oct. 30, 1944 Project Request Memo asking permission to work the design of a new, lighter reel, the Mark 2. This is followed almost a year later by a 1945 memo describing the result, 3C-291, Target Kite Reel. Mark Two, (Crank Bar Type).
Unfortunately none of the supplements mentioned in the memos seem to have have survived, at least copies weren't kept by Garber.
And lastly, here's a tantalizing sketch found on a corner of a larger drawing showing the Mark 3 reel: