I enjoy using the 503CXi with the D-Flash 40 and was looking for a decent grip, since it's quite awkward to handle with the flash mounted directly on the prism.
I bough a nice 45071 grip and to my surprise it doesn't fit the 503CXi. It uses a 3/8" screw and doesn't even come close to aligning with the baseplate hole. I've attached a photo of the 3/8" screw trying its darnedest to fit into 1/4" hole.
On the upside, it fits my 500C/M just fine. Too bad I never use a flash with that body.
So my question is, if I buy a 45072 grip instead will that fit the 503CXi? As I understand it, the screw on the 45072 can be reversed to use 1/4" instead. I'd hate to make the same mistake again.
I'm not sure which grip I have, but it does not use a cable release (the mechanism to trip the shutter button is internal), and I can reverse the screw for 1/4" or 3/8". It fits my CX, and all the 2000 series, as well as a 203FE. I think the 203FE has the same/similar mount plate as the CXi shown above, the other bodies use the older mount plates.
The complete Snap Lock Flash Bracket for D-flash W/hand Strap & Cable Release is part 45169 for the new plate mount.
That has the modern quick release fitted and is correct for the 503.
45072 is the older bracket and does have a 1/4"-3/8" reversible post as does 46330 and will fit your 503.
72 fits 500 and 2000 series and 200 series
30 fits 553ELX and earlier EL
I have not looked but I suspect that will be the most economical route as well.
There was an adapter, not a replacement plate, that fitted over the old style so you could match the bases of the cameras if that was at all important, elusive now I suppose 45148.