Another way of checking: turn the chassis right-side up in the base and insert the 2A3 tubes - the right way will be snug, but doable; the wrong way won't work unless you use way too much force to insert the pins. Once you figure out the right way, check whether the fat pair of pins are farthest from the front, and the skinnier pins closest to the front. If they are, the sockets are oriented correctly. If not, they need to be rotated 180 deg.
cheers, Derek