Of > By
Mulciber > (A surname of Vulcan, god of fire; see PL 1.740 ff.)

5 Some others were new driven, and distent

driven > beaten out distent > extended

6 Into great ingots, and to wedges square;

to > into wedges > ingots

7 Some in round plates without monument;

monument > mark, inscription

8 But most were stamped, and in their metal bare

bare > bore (vb.); or: bare (the vb. "were" being understood)

9 The antique shapes of kings and kaisers strange and rare.