Not sure what a "widget" silver boot is, but if your title is just an error and you just want an explanation of what a silver boot is, I can help! A silver boot is a factory pressed bootleg release - in other words, it's NOT a CD-R. Obviously, before CD-Rs became widely available, most bootlegs on CD were only available as factory pressed. The recording industry cracked down on this over time so that it became increasingly difficult for them to be pressed. Add to that the wide spread availability of CD-Rs and there wasn't as much of a need for factory pressed (silver) bootlegs.
The silvers that are available are likely for the most part from years ago. If it's a silver from a recent show, I would definitely do some research to insure that it is in fact a silver and not just a CD-R that looks almost like a silver. Because they are no longer widely available, they do command a price, altho usually just for the hardcore collectors that have to have "everything". For most bootleg collectors, having lossless (or even lossy) copies of the shows is enough, so the price of silver boots being sold these days is usually gonna be less than they were in their heyday, when they were going for $25 for a single, $50 for a double, or more.
I doubt just about anyone would consider them to be semi-official material, unless perhaps they were soundboard recordings (such as the 6CD Forbidden Dreams bootleg I still have in my collection). The reason why they are noted in MP's tourography database was because when I first started working on it with him in early 1997, silver boots were still pretty much in the minds of bootleg collectors in general and CD-Rs had not gained mainstream acceptance yet due to their pricing.