Stadia staff, patent no.843285 of 1958, to allow tachyometers to be used for large scale work, one prototype, two production models Stadia staff
Circular plotting protractor patented by E.R. Zenthon in 1960, aluminium, 2 ft inner diameter, with 8 aluminium scales and perspex centring sheet in fitted wooden box Circular plotting protractor patented by E.R. Zenthon in 1960