As they say, a picture is worth a thousand words. Now I understand the "spacer" you were talking about. This is part number 3941824 (at least in 1968). It may have been an attempt to help the nuts that lock the swivel in place from backing off. But as for the definitive reason, I do not know.
Spacer is labeled as 11 on page M20 A3 of the 1968 AIM.
On the image of the rod ends below, the rod showing more threads at the end is a 68 big block Muncie 3/4 rod. The other rod is unknown at this time, but possibly 70-72 Camaro small block Muncie.
(Text above changed because pic orientation seems to change based on whether viewed on a of versus smart phone).