What are the characters on this unusual seal (Characters identified: 令憲稽首)
The characters are

which would be expressed today as 「令憲稽首」. This roughly translates to prostrate before the law.

What you have there looks like a seal recorded to be carved by an artisan named 黄士陵. See http://www.ne.jp/asahi/yoshitaka/hanko/kousiryou.html:

The side says

仿彝器文 士陵
In imitation of bronze inscriptions, by 士陵

referring to the script style of the seal being bronze inscription shapes.