Hello,
The 'Big Chief' is a much rarer version of the 'Thunderbolt Junior' grip-cocking, concentric air pistol identical in operation to the earlier German Tell 2.
The pistols were made by Produsit (Birmingham) between 1947 and 1949 under an agreement with Frank Clarke who patented the design. Approximately 8000 'Thunderbolt Juniors' were produced. Reference sources suggest that a small number were marketed by Frank Clarke (Lead Products) as the 'Big Chief'. According to The Encyclopedia of Spring Air Pistols (John Griffiths) only two examples have come to light, both in New Zealand.
If the pistols are indeed the 'Big Chief' variant, they will be marked as such on the breech closure plate (just below the rear site) and would represent an important find.
The 'Thunderbolt Junior' just bears a Pat No impressed into the breech closure plate with no name.
Please keep us updated when you actually see the pistols.
I believe there was also a tin plate, junior rifle marketed by Produsit around the same time which also used the 'Big Chief' name, but unfortunately I don't really know too much about that particular product.
Regards
Brian