In the beginning each railway company produced it's own design of components for wagons.This goes some way towards helping unravel the mysteries of all the varieties of W-iron available as etches from Bill Bedford. Non-vacuum-braked 16T minerals require BWF080. 21T minerals and vacuum-braked 16T minerals require BWF081.
Then the Railway Clearing House(RCH) produced standardised designs which many companies adopted for their own wagon designs, so at the grouping all the big four were using RCH designed components.
After nationalisation BR produced two w-iron designs for their own wagons, an 'open' which was a beefed up version of the RCH design and was used for wagons of less than 20T capacity, and a plate which was used for high capacity wagons.
A new design of w-irons was used for newer air braked wagons built from the late sixties.
BR 16T steel mineral wagons were built with standard BR w-irons, but those that were rebuilt with vacuum brake gear received plate w-irons.
Apparently you also have to be careful which bearings you use, as some are the wrong depth - see Craig W's comment on my last post. I've ordered some Markits/Romford bearings.
As an alternative to the Bill Bedford springing units there's also the Exactoscale ones: http://www.exactoscale.co.uk/downloads/Buffer%20Hight%202d.pdf
Their W-irons come either as a chassis etch with a choice of wheelbase or as W-iron assemblies - see: http://www.exactoscale.co.uk/4mmrollingstock.html#4WC4
1mm bore parallel bearings and spring wire are included.
Instructions for assembling these are at: http://www.exactoscale.co.uk/downloads/INS%201000.pdf
Thanks to Andrew Jukes (of Exactoscale) for this info.