Pete-
When I was a calibration tech 25 years ago I use to get parts through the local industrial suppliers. I'd call them, they would fax me a schematic of the tool in question, and I would call them back with the part number. Also, at that time, Starrett would allow individuals to purchase parts once every six months. I'm sure things have changed dramatically.
That being said, I would think that if you contacted Starrett and gave them the model number you could purchase the lock assembly from them directly.
-Bob