There are several Exhibit card collectors/experts (Adam, Scott, Fred) who can chime in also...but period exhibit cards have cream colored or tannish colored backs.... Grey and White Backs are later reprints....
Here is a good discussion about this from an old thread...started by Scott
http://www.net54baseball.com/showthr...=exhibit+backs
if you do a search function and type in Exhibits...you will also find a lot of great past threads regarding these cards.
Ricky Y