I contacted Interstate Batteries and asked the following:
Is there any difference between the retail MTX-49/H8 and the Costco 65/49-H8-AGM other than stickers and distribution channel?
Thier answer:
Hello Todd,
Thank you for inquiry here at the home office of Interstate Batteries, Inc. The only difference is the part numbers and the warranties.
My guess is Crosshairs body of knowledge on this subject comes from this post on BITOG:
"Tried to purchase a group 75 Interstate Battery at Costco for an Anniversary 1993 Corvette. I went to Interstate's website and found the correct battery application. Went to Costco and was told that they don't stock it nor can they order it. I went to the local Interstate Battery Dealer and met with the Manager. Manager stated that Costco buys Interstate batteries in bulk and get lower tier quality batteries. He further stated that you could only return the battery to the original Costco (No other Costco) for purchase. Interstate batteries do not warranty Costco Interstate Batteries purchased at Costco, per the Manager. Any truth to this? I like Interstate Batteries and Costco has great return policies. IDK?"
In any case, now that this is settled and Crosshairs is proven to be a know-it-all that isn't, sadly time to put him on ignore.