It sounds like a variation of subtracting that I learned in high school (1968). My instructor called it European subtraction.
& 3 & 4 & 2 \\
- & 1 & 7 & 3 \\
You start by saying $9$ plus $3$ is $12$.
You write the $9$ as shown and "carry" the $1$, of the $12$, as a subscript of the $7$ ...
The SMSG textbook for algebra II and trig was entitled "Modern Algebra and Trigonometry, Book 2" by Dolciani, Berman, and Wooton, published by Houghton-Mifflin. We used this during my junior year in high school (1965-66). I wish I'd kept the book instead of returning it to our bookstore for credit, because I recently had to purchase a used (...
Dolciani eventually went on to write an explicit 'Algebra 2' book, used in the 1970's and 1980's commonly. That Algebra 2 book has an exact replica of the material in the SMSG Intermediate Mathematics. So I think that should resolve it. 'Algebra 2' is the SMSG 'Intermediate Mathematics.'