What are the concrete changes the NSA did to the algorithm before DES was published?

Question

It's common knowledge that the NSA, before publication of DES, tweaked the algorithm to improve its resistance against differential cryptanalysis. Schneier writes some of this on his blog, for example ("It tweaked the algorithm, and it cut the key size by more than half.", https://www.schneier.com/blog/archives/2004/10/the_legacy_of_d.html).

However, information about the exact changes the NSA performed are nowhere to be found. I would want to replicate a "pre-NSA" DES variant for research purposes, but cannot find the required information/specification anywhere.

My question therefore is: What are the exact changes the NSA applied before publishing the encyption standard DES?

For the reduction in key size, see the now declassified account. One account of what Lucifer was initially is given by Arthur Sorkin, “Lucifer, A Cryptographic Algorithm”, Cryptologia, 1984. — fgrieu, Apr 18 '22 at 08:33
Did you check the Wiki page? https://en.wikipedia.org/wiki/Data_Encryption_Standard#NSA's_involvement_in_the_design — kelalaka, Apr 18 '22 at 12:18
But was Lucifer the original DES candidate? From the paper it does not appear to be. @kelalaka yes I did check, but the Wiki page mentions nothing of what I originally asked (e.g., what are the CONCRETE changes to the S-box pre-NSA and post-NSA). — performancematters, Apr 24 '22 at 10:04

What are the concrete changes the NSA did to the algorithm before DES was published?

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