Perhaps Dirt Bothers got the frames without a serial number on and added their own 
maybe, but that would count Brens one out if it has a k number. All the old stock skyway ones had numbers already stamped on the bb though, even the unmade tube stock. Maybe DB filled and re-stamped them.
I have what is supposed to be a Dirt Bros 20"...
Curved bridge with washer and K serial number, holes on the inside.
Maybe, once Skyway started to sell the US T/A they boxed up all the imports and stuck 'em in a corner 
Could that be what Dirt Bros used?
Bren 
Could be and some they restamped some they didn't....
Should be a date on the BB too.
no dates on the K serial ones Laz.
We still have one with the K vent holes but with a straight bridge though. Fun, does the bridge welds look ok? colud it have been replaced aftersale? i know it's a long shot....i love it though

on a foot note Skyway seem to have no idea the k ones exist.