I don't know how many M870s and M1100s I have cleaned/repaired in the last 48 years and I have NEVER seen one like yours with 2 holes. You must lead a charmed life...... Bill

Doubly charmed I guess, 'cause two of my 870's have had the two holes.
Remington's help center seems to imply that they were putting two holes in as late as 1986. However I have owned a whole, whole lot of 870's made before 1986 that only had one set of holes. So I don't know, I guess you just have to get lucky.
http://remington.custhelp.com/cgi-bin/re...amp;p_topview=1