From what I read, it's a Korean OEM (Hung Chang Products Co. Ltd., C.P.O. Box 3125, Seoul (South-) Korea. Measurement equipment.)
Tenma 72-720
Elenco MO-1251
Hung Chang OS-620
Neotronics OS-620
JDR Model 2000
A-1 Electronics
AW Sperry 620C
Tenma 72-320
Dick Smith Q-1240
Aron BS-601
Degem System 112
Ramsey Model 2200
KB Electronics Model 33330
OMRON PAN-8022
and now, Ramsey BS-601 (miss labeling with the Aron or they put different model number for the same thing).
I got the calibrations instruction from the Tenma 72-320 service manuel.
I got the user manual for the Elenco MO-1251
I got the schematics from pdf scan .. for the Hung Chang OS-620
I think I have everything now