Your opinion about general location of your mystery card was correct, soon as I saw it I knew it was the Lee Navigation which flows from Hertford down to the River Thames roughly following along the county boundaries between Hertfordshire and Essex.
            I have browsed my own collection of Lee & Stort Canal Cards without any luck, however I have just been looking at my copy of a guide to "The Lee & Stort Navigations" which is illustrated with postcard views, including one of my own. It would appear to be a view of Waltham Common Lock, the guide carries a vertical card circa 1900 which is almost identical to your picture, except bridge over tail of lock is painted white, and lock cottage has white fence in front of it. Hope this helps, you may have more responses in time from other members.
Ian Wilson

            We are pretty certain it is Waltham Common Lock on the Lee Navigation. We have a similar but not identical unposted card. The photo was taken prior to 1922, when the lock was rebuilt to a new pattern. We suspect that yours probably postdates ours by some years judging from tree growth, etc. Ours is possibly around 1905 because it is an early number (711) Charles Martin, when he was at 39 Aldermanbury. Yours shows trees as tall as the house, whereas ours has no trees at all.
Richard Thomas/Dennis Ashby

            The October mystery is a card of the Upper Lea (that is above Enfield.) I am not quite sure which lock it is and haven't had the opportunity to cruise the Lea to look as I am boating in the Midlands. Most Upper Lea lock cottages have corbelled chimneys on both gable ends, which narrows it. I am sure it is not Hertford, Ware, Feildes, Dobbs, Aqueduct, Cheshunt or Waltham Town. I am pretty certain it is Hardmead (just below Ware) but it could be Waltham Common. It no one else knows I will look next time I am on the Lea.
David Adams