I got my answer from Steyr, Austria.

I will put it here in case anyone has similar questions.

"Dear Sir,

thank you very much for your email and for being a STEYR ARMS customer. Yes you are right!
"BTH" was produced in 04/ 1986 and "280" was produced in 02/ 1980.

Freundliche Grüße/ Best regards
i.A. Christine Träger
Sales Assistant

"