WOW! You got one helluva deal on that kit at $24.00, shipping included. I have seen that kit go for $100 or more, at several model contests over the years.
Back in the mid 80's, I was able to pick up the two rifle kits in the photo, from a hobby shop in Oregon that was closing up due to the death of the owner. If memory serves me right, I paid $5.00 a piece for them. They are both complete and unstarted. At this time, I have no intentions of building either one. As far as date of issue, I was not able to find a date on the box or instruction sheet for the Savage 99. On the box and instruction sheet for the Aurora Mannlicher-Schoenauer, the issue date is 1959.
I did some research on the kits using the old John W. Burns The Collectors Value Guide for Scale Model Plastic Kits, the seventh edition, 1999. He states in there that Aurora acquired the Best molds sometime in the mid 50's and issued only three of the kits. These were the Mannlicher-Schoenauer, Savage 99 and the Winchester 94. He also states that the molds for the other three kits: the Stevens Shotgun, Browning Shotgun and the Remington Shotgun, may never have been produced. This could be very true as I have never seen any of the shotgun kits in all of the years that I have been going to contests, swap meets, garage sales, etc. I have friends that are amazed when I show them the kits, as they have never seen them before either.
Hope this gives you a little more info on the kits. Enjoy your great find.
Terry