There have been tapes floating around with this date for years. However, it seems pretty clear that those are really tapes of a portion of 5-19-1966 masquerading as 2-23-1966.
The Taper's Section on Dead.net, however, also featured a similar looking snippet - Jam> Good Lovin'> Jam> Jam - that they said came from the Vault. I am not going to argue with anything from the Vault, so I will leave it to the sleuths out there to figure out if the Taper's Section selection, which does not seem to be streaming on Dead.net but is available out there on the internet, is actually a portion of 5-19 or is truly something from 2-23.
The great mysteries of lost and mislabeled shows is one of the real treats of the Grateful Dead.