Originally Posted by manicmike

I don't have a rocketeer but I think the 400 series are almost identical.
The slide plate in my 401 goes in from the needle side, like a 201.
If you already know this and can't get it on, it could be that the steel spring that guides it has moved. My 401 has a screw underneath the plate, on the case itself, that you screw in to loosen. You have to remove the bottom to get at it.
To loosen it, screw it in (I mention this again because it's opposite to normal screws), move it to the centre or where it will likely hold the plate on, and tighten (screw it out).