Ah, wish I could tell you exactly but I recall it was early to mid 90s. It was very large and included such as gold strikes of all the "silver" denominations that are attributed to the LONDON mint! Also matte proof sets of all the 1927-1953 years, and BOTH 1922/4 LONDON mint satin specimen/proof sets, and proof sets of many other years (so called VIP record proofs). Even at that time with relatively low interest in proofs of George V and VI, all of these sold relatively rapidly at what today would be bargain prices. I was desperate to pull together funds to get what I could as I knew that there could not be too many such hordes out there.
Again what was remarkable is not that many were very rare (which they were) but in some cases represented the sole source of some coins, and these also again London mint issues! Nobody has ever been able to explain to me how in the world that occurred. As far as I know there was no official announcement of these sales and although I can not prove it, am of the understand that these left under somewhat nefarious circumstances with "insiders" walking away with many for but a song.
As a side note it is and was very difficult to get sets from the 1928-1936 years that included TRUE Wreath crowns in proof; parenthetically I regret to say that many of the slabbed Wreath proofs IMHO, as well as Steve Hills, and M. Rasmussen are not actually proofs.