Hi A'af,
Much as I would like to help with a picture, I have no means for doing this, unfortunately. And I don't think you would be very impressed with an old man's photo, anyway!
In any case, I don't see how this would help much and I only mentioned the appearance of the mosfets I have in stock to indicate that if the lettering includes the prefix "2SK", or whatever, this is just *not* a reliable indication of whether these are genuine or not. Jan had earlier suggested that this was so, but Hitachi have obviously marked these devices differently at certain times.
From my point of view, there is nothing which would cause me particular concern over the appearance of these mosfets, but only testing them properly will tell anyone for certain. Like Jan, I think it would be very unfortunate if they turned out to be fakes, from yours and the purchasers' standpoints, but appearance is not what matters here. Maybe the date codes are unusual in being similar, but if both types came from the same source, they could possibly have been made at the same time, but that would be the only concern I might have if I was purchasing these devices.
I have 2 differently-marked types, as previously mentioned, and I noticed that some pics you showed do have the "Japan" wording which is on all of my devices. As I also mentioned, I don't know what significance this is anyway, as all of these very early devices came from Japan in the first place, and it may be that later versions dropped the "Japan" marking because of this. Few (if any?) other devices I have ever seen actually show the country of origin on the front of the component, so this would not be unusual.
As you say, these mosfets are very prone to static damage and HF oscillation which can be destructive, so care must be taken when testing or handling them.
I can only suggest that potential purchasers should initially accept your word for the authenticity of these devices, especially as you have given your assurance that you will refund anyone who discovers that there is a problem with them.
I don't think that there is much sensible alternative for anyone seriously seeking these long-obsolete parts, and as I recently confirmed on the J-Fet thread, I have received my refund from A'af for the dual J-Fet devices which I had previously returned, and so I wouldn't have any hesitation in ordering from A'af again. These Hitachi devices are not easy to come by, and I don't see what there is to lose, except for the nuisance of needing to return them in the (hopefully unlikely) event of them being fakes.
It just happens that I don't personally use these mosfets nowadays, and I have sufficient 'spares' in stock for any likely replacements etc., so like Jan, I don't have the need for any more.