there are loads of festoons 42 mm long @ 21w , the OP wanted
6 volt 21 watt 36 mm long

as I pointed out in my origional reply . the wattage is in proportion to the dia and the lengh

basicly , if you reduce lengh to 36mm , the bulb dia will reduce , and there will be to much heat , and the bulb will explode

I have several alternate solutions
many of the led supliers , ie: classic led , used to offer _ earth , + earth and AC , the ac option has now disapeared , but they are offering universal _ and + earth bulbs , maybe a phone call to clarify
suggestion 2
as per very old bsa bantams having a small battery(in headlight) to feed the parking light , perhaps the OP can look at modern 5-6v or 12v sealed rechargable battery modules , and hide this on the bike (brainwave - why not put it in the headlamp?), using a modern led bulb , the pack should last a considerable lengh of time , and be easy to recharge