I must be missing something. The Backus publication referenced by OP is from 1978, much later than Algol. Maybe the OP means some substantial difference, but I

I must be missing something. The Backus publication referenced by OP is from 1978, much later than Algol. Maybe the OP means some substantial difference, but I don't see what exactly. And indeed, the Backus publication even has a section "14.2 The Structure of Algol Compared to That of AST Systems".