A lathe chuck was included with my recent lathe purchase that doesn't actually fit my (1968) 10L.
5", 3 jaw with a 1-7/8 x 8tpi. From my limited research, it appears that this is an uncommon size used on early SB 10L and 13" lathes. The chuck is made by DE Whiton, and doesn't have a chuck key. The chuck key is a square socket (see picture). It needs a good cleaning, but nothing appears broken.
The price includes flat rate priority shipping to the lower 48.
Message me for higher resolution pictures. If my price seems out of range, feel free to email offers. - Stan
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(ps: the 4 jaw chuck from my previous email is sold)
Incidentally, the 10L never used that spindle thread, the L was always the more common 2-1/4x8. The 10R, 11, 13, and other early midsize SBLs used the 1-7/8x8.
Stan