I have a white or cream colored oval, with a boxed M on one side and on the other side is an indentation cutting the pill in half with an N on one side of the line and 23 on the other side.
This pill NEGGRAM 1 GM
Generic Name Nalidixic Acid
Imprint Code W / N23
Description light buff, oblong, scored tablet
Info at http://www.drugs.com/cons/NegGram.html