Thank you for your reply, However!

How can I tell the age of a Navigational Computer MKIII? not a rare item I know, however a nice thing to own.
The one I have is a MK3 III H REFNo 6B/250.
It is the no lid type and only one winder. Usual AM mark with a P. Says I.C.A.N Calibration on the back. I can usually work out the year of RAF items, this one has me stumped!

I am certain this type of device was used in ww2. I know the Dambusters film isn’t 100% accurate, however you can clearly see one being used on the way to the dams. Isn’t it strange that very little information is available on Google? I don’t even know what ICAN stands for.
I suspect the letter after the MkIII is a clue to the year.