Yes Rich the M20 sidecar lugs were made as part of the frame. That is, a lug at the top of the front down tube, one under the saddle and one on the left side by the rear wheel. I don't know if the post war frames all came with sidecar lugs? Probably! But the difference was that there was a lug each side at the rear to accommodate sidecar fitment for overseas users.
A sidecar lugged WM20 frame is a very rare thing and I think must be ex RAF. Here is the one I had for a while. The body on this one is a VP Viper. A very lightweight aluminum replica of the Swallow 8, (which was steel over ash). The Vipers were made post war by the "Variable Pitch Propeller Co". Ron
email (option): ronpier@talk21.com
Here is another VP Viper that I owned that was attached to my 16H. This one was bought by a forum member as I just wanted the bike as a solo.
And yet another one I had fitted to my WD/CO. These Vipers are quite hard to find nowadays, but I've taken the medication and got them out of my system now!
email (option): ronpier@talk21.com