This week's Palomar Observatory History Photo of the Week was taken in the machine shops at Caltech on October 15, 1935.
It shows three people (I am not sure who they are) with a steel model of the "tube" of the Palomar 200-inch telescope. They are testing the model for its rigidity.
The tube of the 200-inch telescope is what is known as "Serrurier truss". It was known at the time of design that the steel of the telescope would bend and flex as the telescope moved. Mark Serrurier designed the truss to ensure that from any position that the telescope might be pointed the optical elements would remain on axis with each other and there would be no deflections greater than 0.01 inches.
Later, Mark Serrurier would move on to other endeavors and eventually earned a star on the Hollywood Walk of Fame.