The second series has casting number 5454251 on both cylinders but one cylinder has an extra tapped hole for the brake hose fitting.
This sounds like mine, mostly. See below
Another listing refers to 51-53 as having "vertical cylinders" and 53-59 as having "diagonal cylinder".
Mine has 'vertical' cylinders, and...
On 1954's, the short brake line between the cylinders has unique nuts on the flares.
I'm pretty sure the cylinders I got from NAPA are this style. The hole for the cross tube fittings is 7/16x20 instead of 3/8x24 like the ones that came off the truck. Also the holes are angled away from each other farther than the ones from the truck.
I did manage to find fittings after several calls but bending the tubes is gonna suck. Maybe I can find the correct cylinders? I will try Jim Carter again.
**Mine are the early style, casting #5453825. Problem is my NAPA guy says the early '53 shows only 2 cylinders per vehicle. I'm going to call him back and see if the Chevy part # helps.