Originally Posted by
B Scarpia
Judging from the pic the original, it was almost certainly done with linseed oil based finish due to the period and lack of any red/orange tone. I'd simply wipe on Watco Oil with a lint free rag and wipe off the excess according to label directions. Assuming you have left over veneer, try some tests with just Natural, just Dark Walnut and mixtures of both. Keep in mind the linseed oil in the Watco will darken some more over time.
There are many more sophisticated techniques but likely you don't need another hobby. DON'T use polyurethane unless the speakers are going in a Frat House and much beer will be spilled. Polyurethane will be plasticky looking and not match the original low lustre.