Pitts on Antiquities Sales: When is Tref Kosher?

Mike Pitts, 'Antiquities sales: it’s a funny old world', Digging Deeper 2nd October 2014 Mike illustrates a handful of the 300 or so artefacts sold by Bonhams today. He had not read the catalogue in advance and "chose these pieces purely because of their intrinsic interest these were just things I thought looked nice". Among the objects he chose is a "funny looking" neatly lopped-off care-worn Tony Curtis marble head Lot 79 "American private collection, San Francisco, since the 1960s" - not "grounded" before then. There's an Amlash figurine ("Lot 108: Private collection, Switzerland, acquired in August 2003. European private collection, UK and Switzerland, formed in the 1970s and 1980s") and the "South Arabian" Modigliani-style marble head ("Lot 128: French private collection of G.D, acquired in about 1975")also attracted my eye. As Pitts notes:

what’s odd about this list?  [...]  any antiquity which left its country of origin after 1970, with no evidence of having legitimately done so, was illicitly exported. It current possessor cannot be said to have undisputed legal title. Going through the list above, how many artefacts pass that criterion? How many can show they were either exported before 1970, or were legitimately exported after? You have to be careful how you read the “provenances”. Lot 79 (“private collection… since the 1960s”) might sound OK, but who’s to say what was acquired after the 1960s? Lot 81 (“formed between 1965-1985”) could be pre-1970, but equally may not be. In fact, only three [of these] lots come with apparently unequivocal data implying they do not to break the UNESCO convention. [...] salerooms like Bonhams seem to be cavalier with provenance evidence. Surely they have more information about some of these than they have published? Let’s see it, we would all benefit.

Note the stoic calm with which the British archaeologist notes that under the noses of the entire British archaeological community this whole bunch of antiquities was just flogged off even though among the items that caught his eye, very few has anything like the sort of collecting histories they'd need to make them kosher. When is tref kosher for the British archaeologist?