Slightly different explanation via 2 Corrie uber fan site's information:
"The pride and joy of No.13 resident Hilda Ogden were the flying ducks on her ‘muriel’. They have become Corrie icons as much as Hilda herself over the years. They appeared on Hilda’s wall from the late 1970s to the early 1980s. One of the ducks infamously would be seen diving into the sea on the mural, and apparently Jean Alexander, who played Hilda, would check this before filming.
The flying ducks did appear on the show before debuting on Hilda’s wall. Next-door neighbour Elsie Tanner (No.11) had some on her wall and they were seen in the very first episode in 1960 as well as during the 1960s. But it seems that Hilda never bequeathed them from Elsie. In fact, she acquired them from her Auntie Aggie as she told Sally Webster in 1987. Flying ducks were common ornaments in homes across England at the time."