Master cylinder replacement different

You'll notice that the original master has a bolt-on piston retainer. You'll probably want to leave that off so it doesn't just rattle around on the push rod. I know on the early cars ('66 and older) keeping that retainer is desired due to the fact that they use a return spring and rubber boot around the pushrod. On a '73 that shouldn't be an issue....although I still like to see a seal around the hole just to keep the dirt and moisture out.