The date is missing but what is remaining reads "H 4 - 29". I assume the 'h' is the end of the month of March. The date stamped on the postage stamps cannot be read. Having done some research though, it would seem that this device dates from the 1940s. The Ideal Home Exhibition didn't take place in the early 40s due to the second world war, starting up again in 1947 so I wonder if this was purchased in that era. My father lived in London between about 1947 and 1950. I imagine that he purchased it from an Ideal Home exhibition for her.
If you'd like to read another blog post about the Speedweve Darner, have a look at 'Darn It All' by Cargo Cult Craft. For historical information on the Ideal Home Exhibition, you can find a great post on the Ideal Home Show website here