I'm not sure the reverse uniface exists - I'm certainly not aware of one. I do know there are at least 2 of these obverse unifaces as Murdoch III lots 417 & 418 both had an example with the first coming from the L C Wyon collection sold in 1901. However, there was wasn't one in Montagu (1890), who had only a full coin dated 1825. That suggests the ESC rating of R4 is way out because it is highly unlikely Montagu wouldn't be able to acquire something with '11-20 known' given he had everything else!
This coin was ex-Whetmore 355 and Adams 628 based on the latter's notes from Baldwin, however, following his sale it was entombed and attributed to Lingford on the label. At no point do his notes or any correspondence say Lingford, so I think that was either wrong, or somebody knew something Colin didn't as he had close on a hundred ex Lingford coins and you can't mistake L's tickets.
We can say with reasonable certainty that the Barton's Metal patterns were among the last 1824 halfcrowns struck as the die is breaking up.