No longer Friday but wanted to close my part of the thread, though there
is a bit still going on about technical difference between Rockwell and
Brinell type hardness measurements.

As Dennis suggested, I found what I need to make a suitable replacement
"safety" (shear) bolt for this vintage Hi Lift jack at the hardware
store. A 5/16-18 X 2-1/2'' bolt has the same length of unthreaded shank
as the original part. All that needs to be done is cut it down so only
1/4'' of threaded part is left and Hey Presto! I have a replacement.
I'll make two. One never knows.

-- RJS