Rob, back in 2018 I published a book about the Gouvernement’s Noten of the ZAR. This was based on a decade of research in which I monitored supply and pricing of these Noten from every conceivable outlet online, live auctions and dealer sites a.o. On the basis of that research, I came to the conclusion that certain issues of these noten, notably the high denomination Pietersburg issues, were extremely rare and suggested that based on comparisons with other collectible bank notes, these items should command structurally higher prices. Over the following years two things happened. The first was expected, prices for rare issues climbed, sometimes by a factor of 5 or 10. The second effect though was surprising. The offer of rare issues suddenly increased significantly. This increase did not involve forgeries but consisted of genuine items, noten that were presumed lost but turned out to have sat dormant in collections or Boer War scrap albums for years or decades
The effect that supply increases as a result of structurally higher prices may be at work in the ZAR/OVS badges as well. The OVS badge like you showed for sale at the Dutch dealer could be had for around 25 Pound back in 2010. Now same will cost at least 4 times as much, perhaps enough incentive for a collector to, at least, sell off his doubles.
Forgery only becomes an issue when money can be made with it. This would be through the sexing up of existing items (e.g. adding engraving), reproduction of very rare items that command substantial prices (some rare Nazi badges, bugles used in battle, Confederate war flags etc.) or from mass production where the tooling and upfront cost can be amortised over many items. The pricing of the more common ZAR/OVS badges does not economically justify small scale forgery whilst the market does not reflect numbers that suggest large scale production. So while there may be some dodgy badges around possibly resulting from collectors spending a lot of money to fill an irritating collection gab, there is absolutely no ground for scaremongering or questioning the authenticity of badges like the ones contained in Neville’s Relics of the Past box.