Strange JAP cylinder

[Ralf89]

Hi,
Has anyone every seen such a Roadster cylinder with seperate sparkplug hole like the SV sports engines K and U?
Furthermore this cylinder has very tiny but many cooling fins.
This cylinder was mounted on a 550cc Roadster engine from 1927 ( LY/I ). It has the original bore of 85,7mm and a stroke of 95mm, so it is a real 550 roadster barrel. But I haven't ever seen a roadster barrel with seperate sparkplug hole and such tiny fins.
In the pictures you can see this cylinder compared to a 500 roadster KY/W
Thanks
Regards
Ralf

[Rex]

I nothing less than nothing about JAP engines other than they made many more industrial and stationary engines than bike engines, so maybe it's off something like that?

[R]

Ditto.

Rob Saward has written a little booklet about dating JAP engines,
additional to all the pneumatic palava, as research into aussie built versions.
He'd be the chap to ask, although his health may not assist here.