SLOW 15 MPH yellow diamond signs

[bugo]

These pictures were taken in Mena, Arkansas. The last photo was taken on an old alignment of AR 8 while the first three were taken on a city street. I've never seen signs like these anywhere except for Mena. There used to be some more but they have been removed. There might be more signs like this in Mena but I'm not aware of where they are. Has anybody seen any signs like this? Was this design ever in the MUTCD?

[cjk374]

Nice find! I have seen them in other places, but I can't remember where.