Re: 1974 15hp Evinrude. What's this bolt for?
Standard guns work fine; you just push the head over the fitting. You can buy OMC/BRP Triple Guard grease in the tubes PN 775776 for the guns or use aftermarket marine grease. Many companies produce it. I use Pennzoil Marine Premium Plus Multi Purpose Grease or Triple Guard.
Get someone at the shop to show you how to load the gun.