Yes, the fibre washer is OEM.
Yes, someone lost original. Now you need to wonder what else is missing on your lathe.
The reason I originally asked is while the fibre washer was cutting-edge technology back when this lathe was designed, there are now much better options: specifically a needle-roller thrust bearing. There's nothing new under the sun with South Bend mods and Halligan did a nice video about this upgrade years ago.
Check the video's comments as someone posted the McM parts numbers for the two thick washers and the thrust bearing. Double-check the sizes, but you can get the whole lot for under $10. I did this upgrade on my lathe awhile back.
The reason why the bearing is so much better than the fibre washer is that spindle axial play is absolutely critical for parting off. With the bearing you can adjust it to nothing and it will stay that way with no drag on the spindle. Just squirt a little oil in it occasionally.