Oldgoaly, I've got a couple like that also. One is marked P, S,& W, in the corner and on the straight portion "Steel No. 4 Screw" along with two patent dates and a star. The other one just has a "star" cast on the surface along with No. 5
If you look real close on the the thumbscrews, you should see a patent date. But it is very faint. Unfortunately this date, Aug. 11, 1888 is not a valid date. I have a hunch this may have been a patent for the thumbscrew.
On a brighter note, the dates on the body are valid. July 10, 1888 was for the recess in the body that allowed the foot on the end of the screw to with draw flush into the frame, thus allowing the complete opening to be used. This is no. 385,983.
The other date is June 5, 1888 and is for design pat. no. D18,376 for the frame.
Mike