I just bought a used table saw and a previous owner extended the table and in the extension put holes to mount a router underneath. There's a 1.5-2" center hole and several counter-sunk holes around it for mounting screws/bolts. I don't know whether the spacing of those holes is fairly standard across router brands or if I'm going to have trouble finding a router to mount in the existing holes.
They are not standard, but there seem to be a few common patterns.
Rockler's website has a guide for choosing a mounting plate for your router. These are the router models that they support:
- Group A routers: Makita 1100; Milwaukee 5615-5624; DeWalt 616-618; Bosch 1617-1618; Ridgid 2930 Combo Kit; Hitachi M12VC (fixed and kit); Porter-Cable 690, 890, 7529, 97529 and 8529. Note: Current models of the Porter-Cable 892 require at least two screws to be 1/2" long versus 5/8".
- Group B routers: DeWalt DW621 and DW621K; Craftsman Standard; Freud FT 1700E and 1702VCEK
- Group C routers: Porter-Cable 7518, 7519, 7538 and 7539; Milwaukee 5625-20; Triton TRA001 and MOF001
- Group D routers: Bosch 1619-1619EV; Freud FT2000; DeWalt 625
They also offer blanks that you can drill to fit your router if it's not in the above lists.
On the "Tech Spec" tab of that page, they have PDFs that show the pattern for each of the four above groups, so if you measure the spacing of the holes in your table, you may be able to find one that fits.