McB: sorry i haven't done my research on the Prentiss 19.5 vise yet. i think i remember my Prentiss #26 Coachmaker's vise being in a 1923 catalog and it has the pull pin swivel. i thought the flip lever swivel pins are a bit older, but like i say i'm not sure of that yet. i also have a Bulldog with a swivel jaw that has a wrench to loosen and tighten it up located at the base and not sure what date it was made. here is a picture of my Coachmaker's vise and the Prentiss bulldog.
I found this Prentiss Vise Company history which gives their locations over the years and then finally selling to Parker.
Prentiss Vise Company
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» New York, NY, Headquarters locations (1880s-1940s)
PRENTISS VISE TIME LINE
1868, March 17 Patent issue date; issued to Mason Prentiss
1871, April 11 Patent dated
1872 EAIA "Directory of American Toolmakers" (DAT) start date for Watertown, NY factory. [1]
1876-1912+ Contracted with the Bagley & Sewall Company in Watertown, NY to produce vises
1877, Feb. 6 Patent dated
1877, Nov. 1 American Machinist article. Mfd by Hall Manufacturing Co. Office at 23 Dey St., NY
1883 Office at 23 Dey St., NY (dated receipt; "formerly Hall Mfg.")
1893 Office at 44 Barclay St., NY (postmarked envelope)
1903 Office at 44 Barclay St., NY (dated reciept)
1909 Office at 44 Barclay St., NY (magazine ad)
1910, Feb. Office at 44 Barclay St., NY (magazine ad)
1910, Nov. Office at 106-110 Lafayette Street, NY (magazine ad)
1912 Office at 110 Lafayette Street, NY (magazine ad)
1913, April 1 Office at 106-110 Lafayette Street, NY (magazine ad)
1941 Office at 302 Broadway, NY (loose leaf folder; mentions factory at Watertown)
1948 EAIA "Directory of American Toolmakers" (DAT) end date for Watertown, NY factory. [1]
1950s Chas. Parker Co. bought out Prentiss. For a few years, the Prentiss vises continued in production, as 'Prentiss Vise Co., a division of the Chas. Parker Co.'. [2]
References
1: Pracical Machinist Forum (2006-10-15)
2: Pracical Machinist Forum (2006-10-14)
John Ruth: - The EAIA "Directory of American Toolmakers" (DAT) gives dates of 1872-1948 for Prentiss of Watertown NY. "The New York City location was a headquarters or sales office; their factory was in Watertown" - Prentiss is credited with the tradenames Magic, Bull Dog, Rapid Transit, Monarch, Rex, Gipsy, Star, Yankee and Eclipse. They also used patent holder's names Shepard, Bingham, Blake, and Lewis as trade names.