I'm not a mechanic but I'll learn.
Master cylinder adaptor. Square O ring in middle fits onto a groove on the end master cylinder pushrod that you pulled out of the old drum brake master. A small amount of silicone spray and push it right into the back of the new master cylinder. The M body one will have a groove in it for a manual brake pushrod. And the M body fitting threads are SAE not metric.
If your car has small bolt pattern 4 piston disc brakes on it, you can upgrade to the M body brakes and sell those to the resto crowd.
Master cylinder adaptor. Square O ring in middle fits onto a groove on the end master cylinder pushrod that you pulled out of the old drum brake master. A small amount of silicone spray and push it right into the back of the new master cylinder. The M body one will have a groove in it for a manual brake pushrod. And the M body fitting threads are SAE not metric.
If your car has small bolt pattern 4 piston disc brakes on it, you can upgrade to the M body brakes and sell those to the resto crowd.