Definitely a 1/8" square lock wire. Do you have any of these lying around perhaps? I also can't get the rod wiper (seal). McM has something that should work for the rod seal, though it is .050" narrower than the groove, and they have a good looking double bulge o-ring type head seal that should work for the gland nut. I already have their 4" piston seal fitted as I had one on hand and it looks like an exact replacement.
I haven't ordered the parts yet and if there is a seal kit available I would consider it, especially if it included the lock wire. I might even be persuaded to spring for a new gland nut as this one has been slightly damaged by a prior owner with a big hammer and chisel(function is not impaired). Depends on what it cost.
Jim