Jim;

I guess you had to learn (or be reminded) the hard way about that cam...but it's good that you got it squared away...and far be it from me, or anyone else for that matter to make comments about your "dumbest mistake" ...let those who are without dumb mistakes throw the first insult!...but having a single key is just asking for more trouble and as you can see: Expense!

A less expensive (temporary) solution to allow you start and run the car until you find that single key, is to have that mobile locksmith just pick the ignition switch to the IGN ON position (easy for any locksmith, or, you can even try to do it yourself!), then push the retention pin behind panel at 3 o'clock *, and REMOVE the lock cylinder. You can then safely use a heavy straightbladed screwdriver carefully inserted into the slot in the ignition switch, to start, and run the car until you find the errant lone key, have a copy [or two] made, and reinstall the lock cylinder.

...and may I suggest installing a SwEm Start Switch upgrade ** so that you can prevent the well known "Beatles Effect" for vintage Volvo keys...you know...Twist and Shout!

Cheers

* see additional info at: http://www.intelab.com/swem/service%20notes.htm#Body
** see: http://www.intelab.com/swem/swemkits.htm