No. They are half and half.
I agree that it is a rectangular hole and it would then be a pretty safe assumption that the valve rod linkages are also rectangular.
Front end (see post from earlier today above) are round on both the power and valve gear rods within the cylinder castings.
Hope this doesn't make me sound too much of nerd and the following is a bit of an essay I admit. However although not a fitter I have worked on steam engines on and off as unskilled help to fitters for many years and have finally got the hang of it all.
The piston rod (power) comes out to the cross head as a round rod and that is fixed so it only goes fore and aft. The crosshead hangs from a single (or slides between a pair of) slide bars so it stays on the centre line of the cylinder bore.
From the cross-head the power link to the crank axle is via a pin fix knuckle "U" joint at the front to the crank axle. That link rod, the connecting rod, is flat bar. Pet is a two cylinder engine so there are two sets of this gear. On a two cylinder engine the crank setting is 90 deg. It drives to the back axle and the outside coupling rods also transmit power to the front wheels.
To make the steam alternate either side of the pistons in the cylinder a slider is moved to connect variously to inlet and exhaust ports. On Pet the valve rods are also round bar as far back as the expansion link, from there to the crank axle they are also flat bar. The bit I failed to photo for you adequately is how the suspension of that rod is undertaken as it too must run in just a fore and aft motion. It is possible it just hangs off the lifting link but I'm not certain that would work.
The two rods in the set waggle the expansion link as the eccentrics rotate BUT the key thing with the Stephenson gear is only one of the pair controls the movement in the valve rod at any one time the other being in neutral. The die block is moved up or down to select either the foregear eccentric rod or the backgear eccentric rod.
On Pet's left hand side is a big lever that is connected to the bell crank visible just aft of the forward axle springs. Moving the lever moves the crank, that bell crank turns a rod at 90 deg over the frames and attached to it is the lifting link that looks like a straddle crane over the four eccentric rods. That moves the die block up and down to change which eccentric rod is active thereby reversing the engine. Top rod is usually foregear.
On Pet as I stated yesterday the sneaky design dodge is the V configuration you can clearly see in the underside shot taking four units at one end of the valve gear down to three segments on the crank axle.
The cranks have to be at 90 deg as there are two port movements to each stroke (Inlet and exhaust) making four per rotation. The 90 deg setting means inlet and exhaust cycles should be opposed across the pair of cylinders. It also stops the coupling rods getting crossed which is why even an electric powered steam engine has to have the axles quartered. If you look at the photo of the crank axle you can see the 90 deg offset setting quite clearly.
On a real 3 cylinder engine the crank settings are 60 and 120 respectively but on a model you have to quarter.