There's plenty of aftermarket seal kits for these older Cats, and a Bulldog seal kit would almost certainly be available for your cylinders.
Nige has nailed it, the cylinder part number is where you ID what particular cylinder you have and what seals it takes. There are multitudes of hydraulic cylinder part numbers.
See our Cat 955K & 955L Loader Lift Cylinder - Seal Kit, now online. See more.
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I'm presuming the "X" suffix is part of the serial number, and not just added by you, as a number at the end of the serial number, that can't be identified.
The "X" suffix is superflous in Cat parts and serial numbers. It's used as an identifier within the Cat system that refers to the factory where the item was produced, and is ignored when used in the parts or serial number systems.
Your tractor serial number is defined on computerised systems as 85J0875.
If that's correct, this means your traxcavator is an early 955K, built in 1968 (the first year of production) in the U.S.
If the serial number is 85J875X, with "X" being a number between 0 and 9, then it would be a 1974 year of build, and it's a 955L.
What is curious is you state the engine has a 78P engine prefix. The 78P prefix identifies the engine as a 3304 model, and this corresponds with a 1974 year of build.
If your tractor is a 1968 model 955K, it would have been originally powered by a D330 engine with an engine serial number that is the same as the tractor.