HH is an abbreviation for Hollow Handle. Most sterling flatware pieces are made completely of sterling silver. However, pieces such as knives and pie/cake servers often have blades made of other material such as stainless steel. The sterling handles of these pieces generally are 100% sterling but they are also hollow. The hollow part will be filled with some sort of epoxy to hold the blade firmly in place.
Some pieces, such as master butter knives and butter spreaders, may come in multiple versions within a pattern. Some may have hollow handles and some may have "flat handles" (FH). The flat handle pieces most likely will be all sterling, including the blades, whereas the hollow handle pieces may have non-sterling components.
See the post on this blog titled, "Butter Handling Utensils" for more explanation. You can find it quickly by clicking on "Butter Utensils" under "Labels" in the right column of the screen.
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