I was at a junkyard the other day and noticed a fixed up Monte Carlo. The valve covers were gone. I didn't write the numbers down , but I did notice something interesting. Right in the middle next to some numbers I noticed a large stamping that read "S/R". I don't ever recall seeing this on mine. Could these be aftermarket heads?

 

get in touch with world products. they make "S/R Torquers" for 305/350s. It may be one of their heads. Otherwise I am not sure.