What are the characters on this unusual seal (Characters identified: 令憲稽首) Can someonne help me to decipher this unusual seal ?
Many thanks

Side carving of this seal
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.