I started this prior to a break for breakfast and dog walking so there have been a couple of posts before I picked it up again, so I have included reference to Graham's comments.
I once briefly owned this Arnold & Lewis free-sprung, helical-hairspring patent union chronometer. For long-forgotten reasons, I returned it to the seller. I don't think it had a spring-detent escapement, but I never really understood its escapement.
Ethan - this is an example of the highest quality Morton Patent watches that was produced. David Penney has written a very short profile of this series of watches (Miguel's post shows a page as a backdrop in his post). This is a must for those who wish to understand more. Also excellent value at only £15. He illustrates a number of Patent Union Chronometers signed by Arnold and Lewis, and about one of similar quality, he comments, 'This must rate as one of the finest English made presentation watches of the Victorian period anyone could have wished for'.
The profile includes escapement diagrams of the progression of variants.
As Graham has indicated Miguel has not shown a photograph of the roller and indicated this is not one of Morton's variants for the reason he explained. However, the format of the inscription of the dial is exactly that shown on his London Patent Chronometers, which is significant. This is confirmed by David Penney's description of the movement ...
George Morton, then working in Islington North London, probably the Keighley based watchmaker who took out a Patent for a Robin-type escapement in 1856 - see my recent Horological Profile on these watches.
NB: The reason I believe this is because of the change in name from Morton's Patent to London Patent that occurred in this series of watches around 1860. This ties in well with this small but separate series of free-sprung detached lever escapement watches bearing the Islington address, of which this is good and rare example. I have no proof, however, which is why I was keen in my booklet not to make unfounded statements of fact, and I made, and still make, no claim that there is just one watchmaker named George Morton.
The original patent to which David refers ...
The lower diagram shows a form which no surviving examples are known.
The introduction of the frictional rest duplex-type locking was incorporated in Kelvey & Holland's 1863 patent (#2184).
Note that the initial patent did not have a duplex style escape. This is not an essential part of the 'duplex locking' the locking is the interaction between the extended lever (g) and the ruby roller (c). In this early version, the principle impulse (described as 'direct' in the patent) is given by the escape teeth to the impulse pallet (f). Here is an example from my collection
Later versions have the duplex type escape, and I believe this is the form of the escapement in the watch shown by Ethan. So while the roller and lever essentially did not change, the escape did. The difference between the duplex and Morton's variant being that with a duplex it is the escape (locking) teeth that engage with the locking roller like so
while it is the extended tip of the lever that engages in the Patent Union Chronometer. This is as shown in the patent, but in the later version the radial impulse pallet engages with the impulse teeth of the duplex escape and the 'lever pallets' engage with the 'locking teeth'.