Hey Brian,
No it isn't - otherwise the freshly set CO would change when the plug is reinserted.
Its been a while since I worked on these things, but back in the heyday of K-jet, between 30-50% of the cars I worked on didn't have the plugs anymore. While I'd make an effort to find one if mine was gone, I wouldn't fret terribly if it wasn't there - especially on a 911 where the FD leads a pretty 'sheltered existence' way up high in the back of the engine compartment. Gunters tip with old Volvos is good - VW Rabbits and Audis had similar plugs as well.
Oh, and I just googled "K-jetronic+co+adjustment+plug" and found this
at
http://www.autopartsfair.com/volvo-engine_parts/catalog-item-415081.html
or
http://www.theautochannel.com/autoparts/airpartsvolvo/240items.html
for $12.95. Wish I could come up with something with that sort of profit margin...