An author named Leslie E. Card in early volumes of the Journal of Recreational Mathematics (which started its run in 1968) considered a topic close to that of right-truncatable primes, calling sequences that by adding digits to the right in sequence to an initial number not necessarily prime snowball primes.
There are 1442 restricted left-truncatable primes: Similarly, a right-truncatable prime is called restricted if all of its right extensions are composite.
There are 27 restricted right-truncatable primes: While the primality of a number does not depend on the numeral system used, truncatable primes are defined only in relation with a given base.
A variation involves removing 2 or more decimal digits at a time.
This is mathematically equivalent to using base 100 or a larger power of 10, with the restriction that base 10n digits must be at least 10n−1, in order to match a decimal n-digit number with no leading 0.