It is indeed a 191-5 from 1976/77 and it was used in the PT-19 of the time:
Product Engine .049 (0.8 cm³)
Manufactured between 5/1976 and 9/1977 (#191-5, ECJ #28d)
This engine was equipped with a mesh exhaust screen to prevent painful burns. This was the first solution for people complaining about burning their fingers when priming with too much fuel might have caused a small fireball when they applied power to the glow plug. Another (and maybe more important) effect of the wire screen was to limit power and thus speed, facilitating to fly the C/L models. A small and heavy plastic C/L model with a .049 running at full speed can fly quite fast and is thus difficult to control for the beginner.
This particular engine has probably been used in a PT-19 trainer airplane.