One more detail,
It puzzled me that most of the samples broke at the end of one of the buries, instead of at the eye. At first I attributed this to a short taper and/or bury, but upon examining some of the pieces, the tapers were good, and the buries, too. I now think that the cause might have been that the eyes were too tight, so that one of the legs fetched up, and prevented the load on the two legs from equalizing. Next test, we'll be making slightly looser eyes.
Also, if anyone would like to contribute a shackle or two, we could see if individual style or skill affects shackle strength significantly. Sen your pieces to my shop: BTYR, 313 Jackson Street, Port Townsend, WA 98368