The simplest way to see if your freewheel is working is to (carefully) see if the car rolls forward with the engine off when in first gear (brakes off, foot off the clutch pedal)
The freewheel works like the freewheel on most bicycle chain drives, but it doesn't use chains and ratchets. It will allow the prop shaft to run free if it is running faster than the main shaft in the gearbox. This means that you won't get engine braking in first gear and the car will roll with the engine off in first.
It clearly is only on first gear (it is a part of first gear) and uses rollers that run up ramps inside the first gear - there is a picture on p20 of the gearbox section of the 405 workshop manual
it can go wrong - with wear the freewheel can fail to function but that usually means it freewheels all the time and doesn't engage (ie no first) but it plausably could "lock up". the usual failure is "bursting" in which the drive is taken up too violently, the rollers run up the ramps too hard and physically burst the first gear cog. This usually results is loss of first gear altogether or a locked up gearbox (if the first gear pair lock together and prevent the lay shaft from turning) - car should not be driven at all with a damaged freewheel (and not towed)
as to why Bristol did it - who knows - it was used by several manufacturers in various forms (Rover for example) and in this case its really just a replacement for synchro on first - it actually would have been just as easy to put synchro on first when they moved to BW baulk ring synchro - may have been space constraints in the early drum synchro boxes
There were "fixed" first gears in period (mostly in sports gearboxes - eg AC Ace and Aceca) and they have a higher ratio too. There are some modern "four synchro" conversions that replace the freewheel with conventional BW baulk ring synchro
Julian
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