From IBM PC and PS/2 Pocket Reference Manual S41G-5086-00 page 11:
Disp Sync Description
Type Freq V/H
5151 50/16.257khz Mono
>I have access to a couple of old PC mono adaptors. They have 64KB of dram
>and a parallel port. How can I tell if these are "hercules compatible" or
>knockoffs of the older IBM MDA?
>
>Secondly, a related question: Where can I find the horizontal and vertical
>scan frequencies for a PC monochrome (amber) monitor? I need these, if I
>can get a Hercules clone, to install XFree86 on my linux system.
>
>Alan Davis
>--
>Alan E. Davis Marianas High School (Science Department)
>AAA196, Box 10001 [log in to unmask]http://www.saipan.netpci.com/~adavis
>Saipan, MP 96950 15.16oN 145.7oE GMT+10 Northern Mariana Islands