This discussion may be of interest to you.
http://luger.gunboards.com/showthrea...en-transitions
In summary, here are the relative populations of the three kinds of 1913 Erfurts:
Current population range of distinct variations (assuming 23,000 guns total):
> No stock lug, none or added hold open - 8,835 to 10,372 guns
> No stock lug, hold open from factory - 4,545 to 6,773 guns
> Stock lug and hold open from factory - 7,392 to 8,082 guns
By a fairly narrow margin, those with no stock lug and a factory hold open are the least common.
If you have other examples of 1913 Erfurts, I would love to add them to the list, especially those which help narrow down the serial number ranges of the transition points from one variation to the other.