I think H6 is some Chinese generated number. Probably uses Ba20d dimensions without having to refer to its original British design origins. H6 is not a format you will see in western bulb catalogues as far as I can tell. Most of these bulbs are useless in a lambretta cos the lammy headlamp requires a pre-focused bulb to direct the light onto the right space of road. The cheap fleebay ones just project all over the place but are great for blinding oncoming drivers.
Have you any photos of the inside of the headset after fitting?
You can't really do that with the lince as the headlight goes in last and is screwed to the fibreglass cover. In theory the heatsink etc. is not supposed to be any deeper than the three pin connector.