Apart from design of the base being an older model not much I don't think. One thing is the 53 has a 5hp briggs the 21 and 19 have 4hp briggs on them also the cut might be wider on the 53 but im not sure on that?....
Yes I think your right Gizmo the 53 is the plastic catcher version, I only know that thank's to Rodeobob as I've never seen a catcher myself for the 53 as yet.... And thanks Gadge for what the model numbers mean there is not much information on the net when it comes to the older Rover self propelled mowers..