In 1991, the "pre-rookie" fad was still in full swing, and still no one really cared. Despite all the marketing of minor league cards as the best way to get in on the ground floor of future Hall of Famers, the cards just didn't sell. The only minor league cards anyone was remotely interested were those issued in the late 1970s and early 1980s by Cramer, Chong and TCMA. By the time Impel had joined the party, there had already been two years of mass produced, pack-issued minor league sets from ProCards, Star and Best Cards, and they still weren't generating much excitement.
Impel entered the market with two pack-issued 650-card Line Drive sets, one for AA and one for AAA. While nice enough to look at (if a bit boring), and printed on white card stock that was equivilent to what Skybox and NBA Hoops were using, as with most sets of the time, they were hugely over produced and by the end of 1991, most would be marked down to clearance prices. I remember piles of unopened boxes of these cards sitting in the clearance section of Kay Bee Toys in the mall for $2.99. That's three bucks for an entire box. I don't think Kay Bee or Circus World ever really understood what to do with trading cards. They always hugely overpriced them on release (like above retail), and the cards wouldn't sell. Then, instead of just pricing them reasonably, they'd just slash them to clearance prices after a few months. But I digress...
The pack-issued cards (shown below) all feature the minor league's logo at the bottom of the reverse of the card. In this case, the old Pacific Coast League logo because the Portland Beavers were in the PCL.
Impel also packaged team sets for distribution at the ball parks. The team sets came with checklist cards that were not available in packs.
While in most cases the team set cards were idential to the pack-issued cards (including the card numbering), recently I learned that some of the team sets had backs featuring local sponsors.
It doesn't appear that all teams had sponsored team sets. So far I have verified this variation in the following teams' sets:
Indianapolis Indians (Pepsi/Hook's Drug Stores)
Vancouver Canadians (Card Day Sponsors)
I can confirm the Birmingham Barons set (AA) does not have any sponsor logos.
If you know of any other 1991 Line Drive cards that feature sponsors on the back, please let me know in the comments.