Just a couple of thoughts:
1) are we sure it's a catcher?
1a) there's no exit hole for the air so any grass blown is likely to blow back out again, if it enters at all
2) instead of a catcher could it, for example, actually be a hopper?
2a) or a scoop for a particular purpose?
2b) or even just a cover for something?
3) are we sure it goes 'that way up'?
3a) for example, if the aperture was at the top, it could catch stuff that drops and it wouldn't need an exit hole for air.
4) note some of the horizontal marks on the side. Perhaps it slid into something?
4a) this would also explain the flush aperture faces with no apparent hook or other fixing mechanism, which would also enable it to be slid in/out parallel to the plane of the opening
4b) with the aperture facing down it could have covered something, held in place by gravity
However yes it does seem "home built".
Just some thoughts...
