Gayo? sikin panjang with talismans in etched raised relief

[Loedjoe]

I was recently lucky enough to get what seems to me to be a rather interesting sikin panjang at Auctions Imperial (my thanks to Anne and colleagues there for their help).
It has a number of talismans in raised relief (very worn; I have done my best to photograph them; photographs have not loaded in the order I wanted) on the blade, a brass collar and guard (so may well be Gayo), with inscriptions in Arabic, and appears to be of considerable age (possibly 18th cent?), perhaps with a later hilt? The inscriptions underneath the guard read 'There is no god other than Allah' and 'Muha'mmad' (my thanks to Ahmed for the translations), and are provided in mirror image under the other side of the guard. Length of sword 77.7 cm.

The designs on the blade are more or less the same on each side (centipede two cm longer on the right side, for instance), and the designs on the right side of the blade are even more worn away than those on the left.
On the left side, starting from the tip
(1) a round seal or medallion, 1.4 cm diameter,
(2) a scorpion, 2.7 cm long,
(3) a centipede, 8.8 cm long,
(4) two round seals or medallions, mostly rubbed away, the second one apparently the same image as the first, about 2.8 cm in diameter,
(5) on the top two-thirds of the base of the blade, meandering foliate designs, about 8 cm long,
the first 10 cm of the spine also etched, almost all rubbed away;

I have a few questions about it, which I hope members of the Forum may be able to help with.
1) are the seals/medallions on the blade based on the designs of coins or seals?
2) is there a particular significance to scorpions and centipedes on the blade?
3) can anyone tell me of other North Sumatran blades with raised designs?
4) why does Sumatran brass (as in the collar) not seem to tarnish like European brass?
Any other information would of course be most welcome.