All the details you mention are correct. I also cannot find a maker for "HT." This could be due to three things; a misidentification of the maker's initials, a misidentification of the monarch's head, or there really is no information on this maker. It is more likely that the marks have been obscured by time, making them hard to read. There is an "HB" (Hugh Beard) and an "HJ" (Henry Jefferys) from London. Both of those marks could be misread if the second letter is partially obscured, and both marks are also makers from the late 1700's to early 1800's. That would also mean, if this is true, that the monarch's head is not Victoria. You can always try to contact the government assay office in London. They may be able to help you identify it.