Viking 5812

If you've measured the diameter and can find a similar product (even from China) - problem solved, as it doesn't have to match anything for length. The chrome finish may not match but the fitting will be either to a plain steel tang or (more likely) a standard 3 mm square plastic post, virtually standard for old Japanese amplifiers.

Here's an example of a replacement type (check if you can, the outside diameter so it will fit the panel) 2pcs NOS Marantz Aluminum Power Switch Knob Button Fits 3mm Square Shafts NEW | eBay No doubt, you can find cheaper elsewhere by searching a bit more.