True there were never mention of Picard serving on board a Constitution Class Starship. We don't know if the Stargazer was the first starship he was assign too, or if it was another starship, or what class it was? But there still is the model of a Constitution setting on the pedestal in Picard ready room for the first nine and half episodes. Originally the Stargazer was going to be a Constitution. But cause a Constitution had set on the pedestal for the first 9 and half episodes and it no longer represent the Stargazer, but it still represent a starship, but which is not the Enterprise-A. Cause the Enterprise-A never had sideways nacelles, which the model does have.