What is this Mk 3

[remus]

Heres a tricky one for you:

[remus]

Seems a clue is needed:

Its a road safety device.

[pav_3j]

does it warm your tyres in wintery conditions?

that is, unfortunately the best I could come up with.

Pav

[solo]

A wheel lock of sorts?

[remus]

No and No, think bicycles...

[Jim]

a deer whistle. or a wind powered light.

[remus]

@jim said:

...or a wind powered light.

That was a lot quicker than i thought, well done!

[pav_3j]

I have got to get me one of those

Pav

[remus]

Available form all good bike shops

[boofredlay]

Darn it, I thought it was a squirrel laser.

[Daniel Bouchard]

Probably a ventilated bicycle seat, in order to cool down your....