Hello. 4mm unmade kits for the 517 appear occasionally on ebay, it is worth putting up a search. The big turn-off for a prospective new kit is possibly that there were so many variations that there were scarcely two the same by 1930 or so. Some of the differences were fairly minor, but the fitting of outside frame trailing wheels on some to overcome the ongoing issue of hot running bearings, roundtop vs. belpaire fireboxes, and lengthening of the frames from the early version, presents a challenge. So far as I'm aware and as previously said, there have been just the M&L and Gibson ones in recent years - not sure if the Mallard one became one of these. All depicted the locos in later years, if anyone wants to see one in Victorian era, they'd probably need to revert to scratchbuilding. See how many differences there were by taking a look at the RCTS GWR locos partwork.