You put the BG with the high side towards the lead core. The BG punch is flat faced, with an indent in the center of the face a few thousandths larger than the hole in the BG, and about 1/8" deep. As the BG is flattened downward during the swaging process to make the core into a bullet, a little lead is squirted thru the GB center hole, and is trapped by the indent in the punch's face forming a brad that holds the BG to the bullet's bottom. The size of the BG hole and the indent in the punch face depends upon the bullet caliber, but the .45's BG hole is about 1/16" or there abouts... it's not a big hole at all. The biggest benefit of BGs is that there is no need for lub, or lub groves. While the bullet can be pushed a little faster there is really no need for that benefit most of the time.
Concerning other older swaging tools on the secondary market: beware! some aren't worth your money or time. Always ask specific questions such as "what is the brand, when was it made, is the firm (person)still in business, Have YOU made any bullets with the set, are their ANY defects with any part (and if so what are they), is a certain type press needed, are you sure it's the caliber you stated, will you take it back if it doesn't pass my hands-on inspection?). Ebay, Gunbroker, and Auction Arms are sites to see what's available on the 'net. Gun list is a good place to see what's advertised in hard copy. Of course, Both Richard and Dave Corbin list some used equiptment on their sites thou neither really get very involved directly in this business of used/prior owned stuff. And, I've never seen an offer to sell (or buy) any swaging equiptment on this site YET. There have been many folks making and selling swaging tools since 1942, but very few exist today. In many cases the designs were flawed. Several of these poor sets are still floating around and those who own them now have no idea what they are really worth. On the other hand there are a few quality sets for sale, but don't assume anything befor putting your good money on the barrel. Unlike bullet molds, you can't afford to 'take a chance' when it comes to the much more expensive swaging tools. 8)