I think I've answered my own question. This is indeed a Bastin Action aka Bastin-Lepage. As Steve Nash pointed out, Purdey made a number of guns using it. Many other London gunmakers, Durs Egg, etc. also used the action.

Photo from Holts:
[Linked Image from jpgbox.com]

https://www.morphyauctions.com/jamesdjulia/item/3169-394/
EXCEPTIONALLY RARE PURDEY BASTIN SLIDE ACTION PINFIRE DOUBLE RIFLE WITH MAHOGANY CASE.
SN 6756. Cal. 20 ga. This is an exceptionally rare rifle with only 27 slide action guns found in Purdey’s records, 6 of which are rifles. Patrick Unsworth in his book THE EARLY PURDEYS relates that only 4 slide action weapons are known to exist, 2 shotguns and 2 rifles. This particular rifle makes the third rifle known. The slide action locking system is quite similar to that used in the Borchardt and Luger, being toggles. In this instance the toggle is retained in closed position by snap catch at end of long forward hinging lever. It is interesting to note that the tops of the pinfire hammers have holes drilled up through tips to catch the cartridge pins while bbls are being slid forward, and act as extractors. This rifle has 30″ fine Damascus bbls with raised flat rib, file cut at rear behind three folding leaf express sight and at muzzle behind steel bead front. Top of rib is engraved “J. Purdey. 314 1/2 Oxford Street, London.” Bottoms of bbls are stamped with “JP”. SN is on slide. Sling eye is attached to square block on bottom of bbls. Pinfire action has large steel housing for toggle slide system. Peninsula back action locks with rear sliding safety bolts are mounted with tall flat bodied serpentine pinfire hammers.

Last edited by Argo44; 03/29/22 03:48 AM.
Baluch are not Brahui, Brahui are Baluch