Let's move back from the abstract to the concrete. From what I can tell the weight threshold for silver objects didn't exist before the Hallmarking Act of 1973, which is to say a minimum weight below which a "pass" was given for certain light objects wasn't established as a legitimate exemption from previous requirements until the 1973 Act went into effect on January 1, 1974. Since just about every silver object we care about in the pipe world is a) from the UK, and b) predates this Act, sometimes by a century, this is an important point. That's the good news, if one point of clarity is synonymous with good. The bad news is that the 1973 Act was designed to coordinate, update, and codify national standards to replace a large hodgepodge of previous legislation. Sorting through the many prior Acts (some promulgated in Ireland) is an enormous ordeal. Some were highly specialized, but many were designed to cover a large spectrum of markings, literally anything on something offered for sale: patent numbers, design registration, trade marks, country of manufacture, purity of substance, etc. To add to the complexity many Acts were designed to implement one or more amendments to prior Acts.
The foundational legislation in relatively modern times is the Merchandise Marks Act of 1887, afterwards amended many times on many points. Nowhere could I find mentions of weight exemptions for silver; and the penalties for false markings of all kinds were significant enough to be a deterrent.
I should say that the general subject of hallmarks, unlike trade marks, patents, and design registrations, is a rabbit warren of what is clear, what is implied, and what is opaque. For example, what are often interchangeably called makers or sponsors marks should, starting in the second half of the 19th century, more properly be called the latter. The reason is simple: this mark is a statement made to the buyer, and it tells you who assumes legal liability for its accuracy. In other words the "owner" of the mark is on the hook for the false marking penalties cited above. In earlier centuries the Acts contemplated individuals as sponsors, but as limited liability entities were created in mid-Victorian England (the first landmarks are the Limited Liability Act of 1855 and the Joint Stock Companies Act the next year) companies began to become sponsors too. This is why, for example, we continue to see EB/WB marks stamped on silver after both Edwin and William Barling were dead. It's also why we start to see sponsor marks that clearly reference companies (e.g. X & Co).
Much of this was pieced together by me through reading legislation, books and websites that address hallmarking, and engaging in correspondence. Have said that like most highly technical issues a little learning is a dangerous thing, and if we have any IP lawyers among us, preferably based in the UK, their input would be greatly appreciated.
Also I apologize but I don't proof read my posts, I write most of them wearing you beloved gloves covered in all kinds of nastiness. If my many mistakes bother people, just imagine my posts spoken with a thick Balkan accent and it will help for sure 
