Back in the 60's this was done

Back in the late 50's and 60's most V8 engines had breathers welded onto their valve covers.
Why were they only put on the left bank valve cover and if you saw rags tied on them what was this a sign of
updated by @johnny-mallonee: 12/05/16 04:10:12PM

If I am correct Johnny, It was to control the amount of blow-by or keep the oil from blowing out on the engine

if you remember on a chevy the block breather was on the rear of motor--when engine blowby seeped put it seemed to always leak down on clutch causing slip --the rags were there to keep dirt from being sucked into breathers there was also a momentary vacuum during the pulsation of pistons--breather pipes were most always filled with steel wool to further control the flow of liquid out and trash in -- when these breathers were put on right bank oil would gather in valve cover on right side and be blown out breathers --to control this oil being pumped to rockers pipe cleaners were put in pushrods to reduce flow to rockers so buildup in valve covers Paul Zappardino said:

If I am correct Johnny, It was to control the amount of blow-by or keep the oil from blowing out on the engine