This is not going to be a hot tip since I have a feeling there are not millions of '70's T3's on the road looking for ignition keys. For my current project of getting my old T3 running, I doscovered that I only had one ignition key. That would not do - I misplace keys and eye glasses 3 to 5 times a day. A visit to my locksmith got me one duplicate - but it cost $12 (or 2 for 20). I decided to live w/ one dupe.
Fast forward to today. The project is getting closer to the road, but in prep for tomorrow's MG breakfast in Kent, Ohio, I decided to quickly look through my back issues of MGNOC Newsletters - going back to the '80's. I'm going to bring them and offer these newsletters around. Low and behold, I had written 'T3 key, pg 3' on one of the 1984 issues.
It seems that James Mariner in KY discovered that a Master #15 key blank fits the ignition lock "if you file 0.012" off each side". A trip to my basement, grabbed a key, popped it in my key machine and duped my T3 ignition key. Cat's Pajamas! It works...without filing 12/1000 off the two sides. Yes its a bit snug, but it works fine. I'll hit it with that file, but Master #15 or Ilco 1092 (same blank) fits.