That's a good point, Ron.
I was suspect of any small chains working for that reason, but the cord idea stuck in my mind only because an NOS Plate assembly in my spare bits bin, came with a burled nut attached. The attachment was done through one of those holes using that 1mm thick waxed cotton, or linen, cord that shows up from time to time with wartime packaging.
The cording 'might' work for keeping the split pin around for a while, but the other thing I notice on this plate is the edges of the metal are quite sharp and it would not take much load against a cord to slice it.
It's possible these plates were not unique to the 19-Set equipment and these holes had other uses elsewhere. If one was able to pass small bolts or screws through them into something, two plates might make a set of fixed carry handles, maybe???
Then again, not knowing much at all about metal forming techniques, maybe these two holes are nothing more than guides for aligning the plate when the two shoulder pins are riveted into place.
Gosh I hate a mystery when I'm between beer runs!
David