Big, Old Plomb socket

[Northwoods]

Picked this one up yesterday and cannot figure it out.
Plomb (round O) Los Angeles
Part number:  DSP 15
Size: 15/16" hexagon
Date code:  7A  (1927)
1/2" drive
2 3/4" tall / 1 5/16" diameter
No other markings: bands, grooves, ropes; nothing but sweet patina.
I get the 15; it stands for 15/16".  What does the DSP indicate?  And, was it a half dollar well spent?

Mine is similar to this one from AA, except for the DSP mark, the fact that mine is hex, and that mine is much taller.

http://alloy-artifacts.org/Photos/tools/plomb_12dr_dd16_socket_12pt_la_7a_f_cropped_inset2.jpg