Hi
Sorry for short description. They are indeed test CD/DVDs.
On A-bex TDC784 I've found this:
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8
&ved=2ahUKEwj9zOHS173qAhWSyqQKHRHwA8sQFjADegQIBRAB&url=http%3A%2F%
2Fwww.a-bex.jp%2FA-BEX_TEST_DISC_HP%2F2CD_%
2Fstandard_product_tcd_%2FTCD-784_.pdf&usg=AOvVaw2MRu4V0Id30PGjjuJIYleK
On DVD-MJK1 I can't find much info
A lot of manufactures refer to them in their service manuals. Often used for calibration of tracking.
KR
Kim
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