A '50 3100 would have Huck brakes, while a '51 would have Bendix brakes, so there's probably a difference on the back end of the cables. That being said, I went thru three different vendors before I got usable cables for my '51. First I went to RockAuto and bought AC Delco parts they listed, and those didn't fit in the frame clamp (too big), then I bought a set from the Filling Station - those didn't fit either. Finally I bought them from LMC Truck and they matched the originals and fit correctly. I see that LMC lists a "universal" repair kit here but no longer list the complete cables for '51 3100's.
Attached is a parts listing for the cables. Note that there's a slight difference in length between '50 and earlier and '51 and later. But that's not significant.
Jim Carter lists original style cables for 3100's saying they fit from '48-'55. LINK
Attachments
Parking brake cables.jpg (56.98 KB, 33 downloads)

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.