Not too sure really. Given the thickness of the steel plate I would usually say pretty early, but that might just have been a quirk of manufacture. I've never had the privilege of lifting such a heavy sign for its size. Early 1920s??, I'm not too sure.
Anyway, the elderly couple who were the vendors told an interesting story, which I will pass on out of interest :
In his much younger days the guy had worked in an Ironmongers in Stourbridge, and he was told that the owner had, some years earlier, taken delivery of an Anderson air raid shelter just as WWII got into gear, and had taken 3 enamel advertising signs off the shop to hold back a big pile of soil around the newly-built shelter. After the war the owner guy had grown fond of the shelter and, much to the annoyance of his wife, refused to take it down, and hence the signs stayed in situ, protecting the soil mound - there was the Morses, a large Zebra polish pictorial, and another large sign advertising nails, the name of which escaped him.
Eventually the shop owner died and his now widow asked the vendor, who was now an employee, to come take the now 40-year old shelter away ASAP and get rid of the signs while he was at the scrapyard. He asked if he could keep the Morses as it was the best of the three condition-wise, he liked the design and despite the face side being in the mass of soil the composition and well-drained texture had actually preserved the image!
So it spent another 35+ years outside in his garden in amongst their collection of bonsai trees until I came along the other Sunday.