There was a pool of 1,397 M4 and M4A1 Shermans in the UK before D-Day. Were these tanks originally used for training in the United States before they were sent overseas? (Most sources I have come across say that they were built in late 1942 and early 1943)

It also seems that nearly all Shermans in use in Normandy have the later, simpler type of drive sprocket. Is there a reason for this?

Shermans that went into training programs most likely would NOT have had enough time to be turned back in and refurbished in order to fill inventories of the active armored units in the ETO. Remember, the bulk of M4s and M4A1s were sent to the MTO first.

as for the sprockets, it was just a logistical thing. Have a look here:

http://the.shadock.free.fr/sherman_minutia/index.html

When did the six-spoke solid wheel officially enter the production lines? A photo of the M4A3 pilot dated 13 June 1942 shows a mix of both kinds of wheels. The third M4A3 had six-spoke wheels exclusively. I was under the assumption they didn't appear until 1943.