Originally the production did not have an intermission at all so I think that's why people play with where the intermission is. By the time I saw it, around its third year, the production had inserted the intermission where it has been traditionally, after Momma Look Sharp.
It's in the script. After the end of "Momma Look Sharp" and the direction "the lights fade" it says: "End of Act One if to be performed in two acts."
