I'm also thinking that the hole in the nozzle where the cover plate goes might be machined differently on the models that came with the belt lifter. That might be why the belt lifter doesn't fit on yours, because it wasn't made for it. But besides that, you also need the nozzle latch if you're planning on removing the nozzle on a regular basis, and the adapter that I showed you on the other post if you want to use attachments with the nozzle removed.
In my opinion you should just leave it the way you got it instead of trying to "turn your 903 into a 980." You'll save yourself a lot of bother and expense, especially considering how hard it is to find the latch and hose adapter. Either way, good luck with your project.
Jeff