Thanks all for the responses. It helped me find the web page https://naval-encyclopedia.com/cold-war/ussr/whiskey-class-submarines. which shows Project 640 variants of the Pr.613 Whiskeys. A fair distance down the page there is a figure: https://naval-encyclopedia.com/wp-content/uploads/2017/02/whiskey-variants.jpg with the 2nd sub on the right being the closest match I can find. It's labelled Project 640|_| where I've approximated the last cyrilic character as best I can - not sure what it is.