CessnaTPA, do you have the lower profile Longbridge metal bases that the wood frames sit in, as shown in the above post by healeyblue? I believe with the later bases, the cushion just sits within the metal base, and the whole thing is covered with the leather or vinyl, without the wood frame, and then it sits within the frame shown in your post. Those tabs at the back are where the back of the metal seat base clips under and pushes back, in an attempt to keep the set base in place. This is the same approach used on Bugeyes and Mk2 Sprites / Mk1 Midgets . As healeyblue shows, the early metal pans can bolt directly to the floor on the passenger side or to the slider rails on the drivers side. On my early #750 Longbridge BN4, I had sliders on both sides and these were also mounted on cedar rails to lift off the carpet and allow the sliders to work. The seat bases were trashed so I bought later model repro seat bases and then found on eBay a good set or early metal bases as well, so need to decide which to use.
Cheers
Lew