On mine (2005 4/4) the tabs are blocks that fit very closely into rectangular holes in the sliding panel. I know because one came off the very first day I had it (bought used in 2007) as I slid the window open, which was very stiff! The block has a very shallow rebate cut into it, the same depth as the sidescreen material - all very high precision - so it pushes in but won't go through. I debated how to glue it in without making a mess.
I didn't want to use ordinary thin superglue as it would run and mar the perspex. After much pondering, in the end I used "Serious Glue" (EvoStik) which is a thick cyanoacrylate which dries water-clear, and spread the thinnest possible layer on the rebate then pushed it home. The repair is invisible and still in place 5 years later. I fully expected to have to do the same with the other side but luckily it hasn't come off. Mind you I have "eased" the sliding screens by using dry silicone spray (3-in-1 brand) in the grooves.
I think Serious Glue would also be ideal for gluing a conventional block directly onto a sidescreen without a hole, as the glue line has remained completely clear without any yellowing or crazing.
