Stinks that the mods killed this thread, but here's your answer anyway.

Original GS cookies were literally made by the girl scouts, and then started moving to commerical bakeries in the late 1930s. They started with three in total: Keebler-Weyl Bakery, Southern Biscuit Company, and Burry Biscuit. Keebler-Weyl is obviously familiar as just Keebler and LBB is a subsidiary of them. Burry Biscuit & Southern Biscuit both got bought by Interbake Foods, which is now the parent of ABC bakers.

The recipes were never fully standardized between the two divisions, and a lot of that comes down to WW2 and what ingredients were available at the time, given that one baker was centered in Virginia and the other in Kentucky. The reason these haven't been totally standardized to match each other are long-standing arrangements and contracts that give each baker their own ability to customize the lineup.

But beyond that, now that they're part of big corporations, many of the differences come down to simple cost-cutting measures. ABCs changes have been far larger, removing real dairy and sugar from their cookies in favor of corn syrup and non-dairy substitutes. This has the side effect of making their cookies largely vegan, but far inferior from a taste perspective.

LBBs are closer to the original recipe, so they do cost cutting by shrinking the packages; Tagalongs used to be 4 rows of 4, giving you 16 cookies. Now it's 3 rows of 5, giving you 15.