The stock double row 4203 bearing (non sealed as stock and readily available in many brands as non sealed) is 40 mm OD - 17 mm bore - 16 mm width.
The 5320 bearing is 17.5 mm wide but has the other dimensions, that was a bearing I first read about on the Old Britts website and will assume it was mentioned (as a modification) due to them wanting a 2RS bearing (double seals) due to a 4203 being not available in the US at the time (perhaps)
That is what I found here in Australia so did the the machining to use a wider quality brand 5203 (2RS) bearing. (front and rear wheels)
As I was informed this week after inquiring (again based on this thread) a Chinese sourced 4203 2RS bearing is available.
That was not an option for me, then or now but may suit other folk.
I think my wheels ended up at AU$2000 ready to roll, I saw no reason to use cheap no name bearings (which I had purchased via this forum but not used) but that is a personal choice.
If there is a Japanese 4203 2RS bearing as posted here (KOYO etc) I am surprised the bearing house I asked did not mention them.
I can not see a bearing issue in my life time with those I fit especially having a higher specification on the double row and being sealed for all but fit and forget.
It should be noted again perhaps, the 5203 is not a drop in bearing on either the front or rear wheels. (unlike the stock 4203)
If there ever was an issue, quality bearings would be available anywhere off the shelf. (unlike the stock 4203 size in a sealed bearing)
