o2 sensor plug needed

[gherndon at eyeontech.com (George Herndon)]

hi,

i installed my new TT 2.25" exhaust this weekend, replacing the ansa 
model that was not emissions compliant here in NC.

my car is a '78 with a race header that is a 4 into 1 design with a 
bung for an o2 sensor located near the flex joint and cat (unknown 
design).  a visual inspection of the opening shows a fine threaded nut 
in the 3/4" range. my calipers say ~ .74

the autoparts store has bosch o2 sensors for an '81 and it looks to 
have course threads.

can anyone shed light on if they've ever encountered a fine threaded o2 
sensor bung?

ideally, i'd like to plug it, in case i need an o2 sensor in the 
future.  i could always weld it closed and forget it.

thx,

george