does anybody know what this symbol is and what it means?

 

IIRC that just indicates what shift and time of the day the block was cast. To figure out exactly which day it was cast, you'll need the casting date code, which should be on top of the passenger's rear side of the block, and is usually impossible to see if the engine is installed in a car as it's so far back on the block that it's usually under the firewall or transmission tunnel. You'll be looking for a 3-5 digit code that will start with a letter (A-L only, A=January, B=February, etc.), with the rest of the digits being numbers. The next number(s) should be the day of the month it was cast (1=1st, 10=10th, etc.). The last digit should be the last number of the year that the block was cast (0=1990, 1=1991, etc.). Sometimes they'll use the last two digits of the year (92=1992, etc.).