To the best of my knowledge, the sealing washer was used on Smiths mechanical oil pressure gauges regardless of when the gauge was made and what make/model of car it was used in.
As above, you can make them from many materials. The only caveat is don't be tempted to use an o-ring. While the o-ring might work at first, they have a tendency to slip outward over time under the oil pressure.
If you are ever installing a completely new oil pressure tube/line, you can use the generic copper line kits that come with a female 1/8NPT pipe to tube compression fitting. Though the threads aren't the same as on the gauge, a few wraps of Teflon tape and the NPT fitting will work perfectly.