Per Charlton 65th edition, p. 195, for 1950-52:
"The so-called Arnprior configuration consists of 1 and a half water lines at the right. Any trace of the bottom water line disqualifies a coin from being Arnprior."
For 1955, the Arnprior configuration is two and a half lines to the right (Charlton, p. 198).
Your dollar looks like a short water line variety (1952 SWL) to me.