Different kind of M13 primer
This dates back to the 1980's when these M13 primers were first cloned.

Heidecker G, Messing J, Gronenborn B. 1980. A versatile primer for DNA sequencing in the M13mp2 cloning system. Gene 10(1):69-73.

In this paper, they discuss the construction of a primer for sequencing of the M13mp2 phage vector. An EcoRI/AluI digest releases a 92 bp fragment from a portion of the lac operon. This fragment contains what are now called the M13 forward -21 and -40 primers. I do not know who first named these primers but, counting from the centre of the EcoRI palindrome, I think it's clear where these names came from: