I think that the REAL Jack Daniel's (pre-Pro) was probably much more like the brand Lem Motlow put out as his own. In a world of non- or "not very"-aged whiskey, the Lincoln process makes a big improvement in drinkability, and Lem Motlow (brand) added about three months of barrel aging to that. Other brands of that era also offered "quarter-whiskey", meaning three months (a quarter of a year).
The Jack Daniel's, and also the George Dickel, we are familiar with is really bourbon, run through the Lincoln process, and aged the requisite (only for bourbon) four years or more in new oak barrels. I don't believe it was ever like today's single barrel. There was once a special bottling, available only at the distillery, called Barrelhouse #1, which was the precursor to Single Barrel. Made pretty much the same way (i.e., from barrels stuck up in the "rooster's perch" on the top floor of the warehouse), it was an answer to the (perhaps over-)aged whiskeys that were all the rage when it was introduced. I don't think they've ever realized just how good a whiskey it really is.