Those numbers sound like casting numbers. The tractor's serial number is on a tag either on back of the rearend or on the left side of the frame by the brake pedal. Drain the rear, wipe out any gunk, clean it with brake cleaner, and then put it all back together and refill. If you have it apart that far I would pull the axles out and put new axle seals in it.
That buildup on the exhaust valve spring is probably due to a worn valve guide. Exhaust gases get blown through the space between the valve stem and guide and cook the oil on the valve stem and spring, causing that buildup.
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