I may have found the answer to my question on those saw inserts/bolts. I found another tin of coins and relics in one of the sheds, and there were 3 more of them in it. These are Taylor Brothers, also made in Sheffield, England. One has an intact threaded shaft on the end of the square shaft, so they must all have been made threaded, but over tightening them, or, just wear and tear may have caused them to shear off. The smooth ends on the broken ones could have been caused by flexing when in use, etc. You can see how they have been twisted. These are unusual, as they have the old Australian Coat of Arms on them, with the emu and kangaroo looking backward, while the later one has them facing the shield. wiley.