On my Tech V the serial number is on the accessory shoe and on the body under the accessory shoe. Remove the finder sight and see if it's there.
That must be a really early Tech III if it doesn't have a Graflok back. There are Rollex backs that use the Graflok slides going way back (I have a 6x6). Those bellows clips also are similar to what you usually find on the earlier Tech II. I'm not sure when they did away with them.