The ZB theory may not be out the window entirely. The Czech factory also produced loaders for the early British guns, as well as for the .303 Brens they sold to other countries. I believe the British loader was copied from the Czech loader, so the feet/no feet could have come into production from either country.
In the attached photo (photo credit to Kev G from the milsurps.com forum) the top 3 loaders are:
B - Czech made loose rounds filler .303
British
for British ordered ZGB's with 30 round magazines and Czech export Brens in .303 (note 6 metal dividers,6 stokes = 30 rounds/1 magazine)
D - Czech made 8X56R Mannlicher loose rounds filler for the Bulgarian ZB39 (note 5 metal dividers,5 strokes = 25 rounds/1 magazine)
E - British made Bren MK2 .303 loose rounds filler (6 dividers)