Custom machined exhaust emission  port plugs.  Eliminates the funny exhaust chirp caused by removing the exhaust port tubes.  Excellent US-made CNC lathe parts machined from AL and brass. 
 
Only a couple dollars more, but these won't rust, seize and look crappy like the cheap ordinary steel parts offered by the Northridge el cheapo parts peddler.
 
These are the best, You get what you pay for!
 
(These are not pipe fittings, these are the proper threads for the cylinder head)
 
No Chirp, great quality!