I have never seen definitive proof of that, Dustin. 1949 cards had "An Exhibit Card" on the bottom:
See the Paige on the left. I do not recall seeing a Robinson with that notation. He was definitely in the 1950 and 1951 runs; here are two salesman samples with checklists from those years:
(this one is a 1950 because it has 1949 ROY Don Newcombe and the 1949 pennant winning teams; the 1948 pennant winners were the Indians and Braves, so the 1949 checklist would have had them listed)
This one is a 1951 because it has the Phillies on it (1950 pennant winners).
I don't have ESCO issued checklists for 1948 or 1949, so if you have something, please share it.