Hello, Dan !
Slava is right about not much information survived about Lend-Lease "Harleys" in Russia. But all is not so bad.
1) The Soviet Union has received 26.670 examples of "Harleys" WLA Solo motorcycles in total under Lend-Lease agreement.
2) The images of Russian WLA with sidecars are not too rare:
Here are Soviet column in Romania (August 1944).
Further two pictures shows Russian WLA with sidecars during the Soviet - Japanese battle campaign on Pacific in 1945:
The Russians used on their "Harleys" the sidecars of Russian design only from Russian M-72 motorcycles (the pictures above shows such examples).
Let's go further. Russian documentation on "Harley" WLA exists only for Solo motorcycles (in open access, at least). But there are many documentation on WW2 Russian M-72 motorcycles (including sidecars, of course).
So, I think - if the Soviets used on their "Harleys" the sidecars from M-72 motorcycles, the sidecar's fixation system on "Harley" was same like this system on M-72 motorcycle.
And the drawings of sidecar fixation from M-72's technical Manuals will be useful for You (to understand an Idea and principle of fixation, at least). Moreover - I feel sure, the fixation systems on "Harleys" and M-72s were absolutely same.
And if You has not lost interest to this subject - send me PM with Your e-mail address, and I shall help You with some drawings from M-72 Manuals.
Regards.