This is what I heard at an exposition of hundreds of relics at a local parish – it is by no means authoritative – but I’ll share it here anyway…
An object which is intentionally touched to a relic becomes a third-class relic. It could easily be understood that the thecla is, therefore, a third-class relic of the Saint whose relics are encased.
An object which is intentionally touched to a third-class relic becomes a third-class relic. Therefore, if you touch something to the thecla, it becomes a third-class relic of that Saint.
You absolutely MUST NOT open the thecla. It should have a seal on it that verifies its authenticity. If you open it, the authenticity is lost forever, because it could never be verified authoritatively that you did not replace the true relic with something else.
God bless you!
Gertie