The bolsters were/are supposed to be a reinforcment to add strength. They are located around the root(gusset) where the standing breech meets the water table(action flat). Basically they were trying to make it stonger by adding more metal. Whether or not they do anything is again an object of some debate. As an example read 400nitroexpress's post above.

As far as throughbolt or not a lot of it depends on who the maker is and when it was made. Some use/used them however I beleive most were not.

Bill