First of all, thank you everyone who has replied so far.
One correspondent claims this is a Spanish Netherlands coin. It certainly looks very like some coins from the Spanish Netherlands. In particular, the design of the Brabant 1 Korte of Felipe II (1569-79),
https://en.numista.com/catalogue/pieces105158.html
This design closely matches the design on my coin (3 fleur-de-lys and 3 sloping bars), but the inscription (DOMINVS MIHI ADIVTOR) and the size and weight are all different.
A design feature on side A (the shield side)
(1) Practically all Spanish Netherlands coins show a crown above the shield whose arches are decorated with jewels. There is NO such crown on my coin! Instead, a central curlicue replaces the crown. See my drawing of this device below:-
There are 3 tilde-like devices to the right of the curlicue, and possibly a similar arrangement to the left, though only blurred marks now remain on this side.
After looking at all the coins listed under “Spanish Netherlands” on the en.numista website, just about the nearest match I can find to this curlicue is a 1 Korte coin for Gelderland - but everything else on the Gelderland coin is totally different from my own coin.
https://en.numista.com/catalogue/pieces85065.html
None of the Brabant coins illustrated on en.numista’s website show a curlicue.
Inscriptions
On side A of my coin, about 75% of the inscription has been worn away. On side B, about 90% of everything has been worn away!
(i) On side A, starting above the right-hand curve of the curlicue, approximately the first quarter of the inscription can be read, which might read:
(a) BON*V BRO... [or BRV…]
(b) BOVIV BRO... [or BRV….]
(c) BOMV BRO….[or BRV….]
* the‘N’ symbol is written in reverse with the diagonal line sloping UP from bottom left to top right.
NONE of these readings looks like: DOMINVS MIHI ADIVTOR (the inscription on the reverse of the 1 Korte of Felipe II.
(ii) Still on side A, if the coin is rotated 180 degrees (so that the shield on the other side now points upwards), another string of letters appears: VON*E[?]V…..[with the same ‘backward’ written ‘N’ noted above.] The remaining inscription has been worn away on this side.
On side B, what appear to be the letters DIOR occur at a point corresponding to the tip of the shield on the other side. Everything else on side B is either worn away, or perhaps only readable to a trained palaeographer or an expert on Spanish Netherland coins.
Someone asked if there was a figure standing on side

But as the blue-coloured photo above shows, much of side B is too worn to be decipherable.
The physical details suggest this is not a Brabant coin. I’ve weighed it twice using different scales but it still weighs 4.5gr each time. The weight plus the diameter (27-28mm) don’t seem to fit any of the Brabant coins listed on en.numista.
So instead of being a Brabant
coin, could it be a Brabant
token – perhaps
a jeton? Or
a Nuremburg jeton, as someone else has suggested?
ozmy19