Now Bowman had no counter to this move; they had no supplemental sets to offer, metamorphic, metal or otherwise. Their only two real side issues were the1949 PCL set and the Black & White cards that rounded out 1953 when the Kodachrome stylings of that year got too expensive to justify.
So why did Topps withhold the last 30 pins? I suspect it was because the battle was won and when they agreed to purchase Bowman in February of 1956 (a deal that would close on April 1 of that year) there was no longer any need to spend the money on a last series of 30 pins so they just stopped production. Topps would occasionally do this for various reasons but in '56 I believe they were stretched on the cash flow front due to the purchase of their primary competitor and just decided to can the rest of the set.
As a bonus, when I was researching this post, I came across a scan of the box for the Pins (actually they were called Buttons) that I did not have for my previous post on the set. Check out this bad boy:
That candy coated gum almost certainly represents the death throes of of Block Busters gum, a product that was fizzling out at the time and which resembled Chiclets. Once again Topps was able to take a failure and repurpose it. I would not put it past them to have taken returns of Block Busters and taken the gum nuggets from those packages and inserted them into these.
The bottom is quite informative. We learn that the set was definitely was to have 90 subjects.
The Bazooka premium catalog offer is a nice bonus! Personally, I consider the '56 Pins to be one of the best supplemental Topps sets ever; it's just too bad it never could get past 60.