Graham,
I used to be involved in the annual testing of winches on military equipment, ( recovery vehicles, trucks, plant, ARV, etc) while working for REME, also mobile cranes and other lifting equipment. The safety factor for winching tackle is 2:1, hence the difference in size of shackles, pulley blocks, etc. as you pointed out. From memory, last involved about 13 years ago now, was that shackles, pulley blocks, snatch blocks, and ropes are tested on manufacture and certificates issued, after that, the periodic inspections are carried out, in the Army it was annual and on the items I mentioned, are visual only, but if the original test certificate is missing, lost, etc. then the item has to be scrapped as it has no history. There would be identifying numbers on item and certificate. We had to test the winches on full load pull to check cut outs and warnings, also correct functioning, ropes were inspected before and after this test.