My memory, many years ago, of the Mamod whistle is that being small, it was high pitched and "screechy".
LBSC showed drawings of the whistles that he fitted tom his models, when he wrote in Model Engineer, back in the 40s and 50s. No doubt they will be available more recently, probably in the drawing packs for his various models.
Hopefully, with details of how to tune to get the required note..
Howard