Tony, it's a Mont Impala. The impala was their bread & butter for years.
Montesa were the creation of Pedro Permayer who, following the Spanish civil war which left the Spanish economy rooted, saw a market for a cheap utilitarian motorcycle. Permayer enlisted the skills of a very talented designer and together they designed the impala.
In order to expand their market Permayer and his designer began racing their bikes, slowly breaking into the off road scene but concentrating mostly on the road racing side. In the early sixties Permayer decided to quit racing. A decision with which his designer disagreed vehemently. He left Montesa.
The designers name: Franceso Xavier Bulto
(P.S. it's a 175)