"Numerical Palindromes: Part 1", by Susan Thorpe in the November 1996 Word Ways, dealt with what I described as numerical palindromes of order 2. It was her work that led to my own effort.
"Numerical Palindromes: Part 2,"
Word Ways: Vol. 30
, Article 21.
Available at: https://digitalcommons.butler.edu/wordways/vol30/iss1/21