Is there a history to the custom?
USPC has a history of doing this from the beginning. It's all just model numbers, for tracking different types of cards. They have separate numbers for the different card backs they've used.
In
this post on Sprouts1115's deck project for making a 64-card deck with four suited jokers, I gave him a list of numbers currently or at least recently in use by USPC - there are many others in their history. Here I've updated it with a few recently-used additions, some of which may no longer be in print.
7 - Streamline jumbo index
9 - Tally Ho (both backs)
11 - Streamline
21 - Streamline pinochle
44 - Bicycle Rider Back pinochle jumbo index
48 - Bicycle Rider Back pinochle
77 - Bee jumbo index
68 - Bee bridge sized (old number, no longer in use)
86 - Bicycle Rider Back bridge-sized
88 - Bicycle Rider Back jumbo index
88F - Bicycle Rider Back jumbo index Braille
92 - Bee (named after the year of initial release, 1892) (also used for modern bridge-sized decks)
125 - Bicycle Rider Back "EZ See Lo-Vision"
606 - Congress (all backs, standard index, bridge-sized)
807 - Bicycle Rider Back in "classic" box (AoS still labeled 808)
808 - Bicycle Rider Back in new "Standard" box
809 - Bicycle Mandolin Back
813 - Bicycle Maiden Back
914 - Aviator
917 - Aviator jumbo index
918 - Aviator pinochle
999 - Steamboat
1201 - Hoyle
1202 - Hoyle jumbo index
1205 - Maverick
1206 - Maverick jumbo index
1211 - Hoyle pinochle
Tigers were indeed 101, and there were the Army and Navy decks, which were number 303. I know there's more out there.
On occasion, a manufacturer will chose a number for a specific reason. For example, the original Legends release was numbered 852 (if memory serves) because it's the area code for Hong Kong, the city where the company is based. And as mentioned above, the model number for Bee playing cards was the last two digits of the year it was released.