Hawkman - The two plugs on the back serve different purposes and are supposed to be different designs as well.
Looking at the rear of the block, the plug on the lower right is in fact the camshaft bearing plug (GM# 3704158 - Plug, cam shaft bearing: 2-1/8" diameter x 1/4" thick cup style) which fits 1954-62 Chevy 235 and 261 engines, not to mention most other Chevy engines. It's exposed to the oil lubricating the rear cam bearing and not unusual for sludge to be found back there if the engine wasn't well maintained.
The plug in the upper center of the block is for sealing the coolant passage core opening (GM# 3835121 - Expansion plug - drain, cylinder block (front and rear 2" diameter). AKA - "Welch Plug" which is not a cup design, but a convex disc that expands to seal the opening when struck in the center which expands the diameter to seat/seal against the core opening. The sludge behind that plug shouldn't be oily, but coolant can have an oily/slippery feeling.
I recently posted about the plugs in a couple other threads, noting that Dorman offers a replacement for the 2" coolant core plug as
Part# 550-028. That listing is for a package of 10, but you can purchase them from your local FLAPS for around $2 or less each. Note: By 2 as these fit both the rear and front core openings on your block.
For the camshaft bearing plug, Dorman lists the replacement for GM# 3704158 as
555-049. Also available from your local FLAPS.
