Black KH,
Don't confuse star hubs with K hubs, they are different creatures. From the pics of your bike the rear is later Sportster ( from the 70s!) so a K parts book will be incorrect for it for '55, yours is like a '56 and later type. Your hubs are dead easy to service. I would recommend NOT filling with a grease gun. Take the wheels off, remove, clean, and inspect the bearings. They are all ball bearings, front and rear. If they are good re-pack them with high grade grease, re-assemble them and you will be good to go for many thousands of miles. If you service them regularly (every 20K or so) you will never need that grease gun. There are no shims, adjustments or any other issues to think of. The center spacer and the retainers are all there is. Bearings are available from any bearing house or good auto parts store. You should be able to do it all in a day even never having done it before.

Robbie