Looks homemade.
Maybe the original sight was lost and the owner decided to attempt something better.
I not sure he succeeded - the original sight is smarter than it appears, the windage sledge is in a tapered groove and it doesn't move unless it's tapped free from the taper. This allows the elevation to be adjusted easily without affecting the windage.
Although I agree the replacement sight looks well made, a simple, uncluttered sight picture is important for me and as stated above, sufficient adjustment is possible in both planes using the original rear sight arrangement and once set rarely needs further adjustment.
Brian
A bit Heath Robinson I would say.
Baz
BE AN INDEPENDENT THINKER, DON'T FOLLOW THE CROWD
I quite like it.
Bru
Webley Mk3 x2, Falcon & Junior rifles, HW35x2, AirSporter x2, Gold Star, Meteors x2, Diana 25. SMK B19, Webley Senior, Premier, Hurricane x 2, Tempest, Dan Wesson 8", Crosman 3576, Legends PO8.