Let's stay in the '60's, before digital images, and for most states (probably all) computerized certificates didn't exist either.
You made the statement that there's "no such thing as an original birth certificate."
The "changes can be made to a birth certificate" don't affect the "original birth certificate" (that you argued doesn't exist).
Changes could be made, as any new parent who received a photostat of the original will tell you (since it's accompanied by a letter urging parents to correct any erroneous info) -- but those "changes" (if approved) will only appear in the computer-generated printout, if they fall within the range of the (abbreviated) data printed out on those. (And the "changes" usually involve misspelling).
The original -- the document submitted by the hospital, remains the same (that has changed recently with computer submissions, but we're talking about the '60's here).
I have no idea what you're talking about in regard to documents that "had to be signed by a computer (or machine signed) back in the '60's."
I somehow doubt that the state of Indiana was so far advanced in the '60's that all their certificates were "machine signed" and "computer generated."
Can you provide a link or some proof to that effect?
The purpose was simple ~ to get a Social Security number. Back then you didn't need one to get a bank account, but you sure did need one to get into college or work for a major corporation.