What is the 300 amp maximum discharge current for?

It is a ceiling for brief current. Think motor inrush, a switchgear trip coil, or the first instant of a transfer, all of which ask for a large current and release it within seconds. Sustained output is the watts column: 90.0 watts per cell over fifteen minutes, 179 over five. The 300 amp figure tells you what the terminal and plate group will pass without damage, and it is the number to check when a load has a startup surge that dwarfs its steady draw.

Why does this size bolt down instead of using a Faston tab, and what torque does it take?

Above roughly 50 watts per cell a push-on tab stops being a sound connection, because the contact area cannot carry the current without heating. CSB switches to a threaded lead alloy insert at this size. It takes an M5 bolt at 26.4 lbf-in, with a tolerance of plus or minus 5.3. Use a torque wrench and check the joints at the same interval as the ohmic readings. An undertorqued bolted connection runs hot and reads as a failing battery on a conductance tester when the cell itself is fine.

Four of these in a 48 volt string: what does that deliver?

Four blocks in series give 48 volts nominal and 2,160 watts over fifteen minutes to 1.67 volts per cell, or 4,304 watts over five minutes, at 58.2 lbs of battery. Current stays what a single block delivers, so 48.4 amps at the fifteen minute rate. Series connection multiplies voltage and leaves current alone, which is the arithmetic that catches people out when they size from a watts figure and a current limit at the same time.

Searches for a 12V 23Ah battery land on this page. Is this the right one?

It depends on what the 23 Ah block you are replacing was doing. This is a high rate design: thin plates, low resistance, built to empty in minutes and then float for years. A general purpose or deep cycle 23 Ah block of the same size is built to give its capacity back slowly and to survive repeated deep cycling, which this one is not intended for. If the old battery came out of a UPS, an alarm panel or switchgear, this is the right family. If it came out of a scooter or a trolling application, say so with your quote request and a specialist will point you at the right range instead.