"This Volume Free" printing
This is rather hard to find (but once again, that doesn't mean it has much value, as the Bobbsey Twins are not very collectable currently). Finally in 2016, I was gifted my own copy from my friend Betsy (of the fantastic bookstore in Savanah: Books on Bay). If you're in the area, definitely go check out her bookstore! Lots of vintage children's series books, among many others.
Around 1959, Grosset & Dunlap ran a promotion with some of its series' (Nancy Drew, Hardy Boys, Bobbsey Twins, Judy Bolton, Dana Girls, Tom Swift Jr., Cherry Ames, Rick Brant, Tom Corbett and Chip Hilton (maybe more)), which had stamped on the cover of the 2nd book in the series, "This volume FREE with purchase of (insert series name) #1".
Around 1959, Grosset & Dunlap ran a promotion with some of its series' (Nancy Drew, Hardy Boys, Bobbsey Twins, Judy Bolton, Dana Girls, Tom Swift Jr., Cherry Ames, Rick Brant, Tom Corbett and Chip Hilton (maybe more)), which had stamped on the cover of the 2nd book in the series, "This volume FREE with purchase of (insert series name) #1".