Like the seller, I have never seen this listed or mentioned anywhere before. It doesn't have a number, although the sticker says "Series 2", so there may be also a "Series 1" set in a collection somewhere ...
BTW, final price was £633 ...

Some pics from the auction - https://www.ebay.co.uk/itm/292941599682:
Description from the seller:
Here is a Corgi box for a gift set - the "Special Treble Agent Set, Series 2" - it is an EMPTY BOX - NO VEHICLES included - I'm calling this out as mega-rare based on the fact I've never seen one before and I've been collecting for many years, nor could I find any mention of this set either on Google or on the usual Corgi enthusiast websites I refer to when needing info. The box is 160mm square (just over 6 inches), it has two slits in the lid, the edges of which a previous owner has reinforced with sellotape - no idea why, no idea why the slits are there come to that. Condition is as my photos.
This came my way a few years ago and at the time had the three models in it but was evidently a put together set as the cars were in very different states of wear, the Batmobile was very nice and is in my own collection to this day, the Aston Martin was poor and I sold it on eBay for spares and the UNCLE Oldsmobile also got sold as I had a couple of better ones in my collection at the time. I'm sure someone could put together a nice set for this box which was always my intention but a project I never got around to completing as these days I'm culling my collections, I'm reluctantly going to let this go.