Anyone have the episode on which
Spring Byington of DECEMBER BRIDE
portrayed herself?

Here is that episode.

I have it as probably alot of others do.IIRC 7.5 out of 10 on the VQ scale.

Episode 52. Dennis' Birthday
First aired: 2/19/1961
Mr. Wilson puts on a magic show for Dennis' birthday and asks Spring Byington to be his assistant.