Variant?Marked?Butterfly

[Ed]

During my previous trip to Wulong, Chongqing, China in October 22 I collected this specimen of Symbrenthia niphanda, upon inspecting it once I arrived home I noticed a silver patch on its lower FW.
I am unsure if this is common as I've never encountered such a variation.
Could it possibly have been marked for some type of research or is it likely just a variation?
Thanks in advance for any input!

[Adam Cotton]

It looks as though the scales have been rubbed off, possibly by friction with the costa of the hindwing as it flies. Is this only present on one wing, or the underside of both forewings?

Adam.

[Ed]

It is only present beneath one forewing, the other forewing shows no sign of this.

[ciervo]

Ive marked butterflies in the past for study.
Makings were at the base of the forewings with scales rubbed off and replaced with coloured paint. It was only practical to do this at the base of wings without damaging the butterfly's wings.