Norton Villiers DO7 Engine dating

No one seems to know when these engines were made yet it must have been for a short time I guess some time on the 60's.
Mine is fitted to a Ransomes Sixteen Lawnmower again I can't date this and no-one has been forthcoming with any suggestions.
Are there any experts out there in the ether?
Any clue would be appreciated.

Dating to between mid 1960s and late 1970s.......

Thanks for the response, now need to track it more specifically if that's possible.

The Old Lawn Mower Club may be able to help with dating the Ransomes Sixteen production dates.

Have tried there but no luck so far.

Yes, they are a funny bunch !! I have an old Greens lawnmower and they suggested that it was not worth saving and I should just remove the engine and scrap the rest of the mower.....

Hey Ho that's how it goes, but mowers are like Art. If YOU like what you have then that's all that matters. Variety is the spice of life. That's about as far as I've got with Philosophy, will now have to start reading chapter 2. On second thoughts at 68 do I really care. Sod the lot.