Hold-open fitted after 1913

[A.Mifsin]

Hi All,
On page 31 of Jan C.Still new book Central Powers Pistols It is stated that in 1913 Lugers that were not manufactured with a hold-open were retrofitted with a hold-open by Erfurt rifle Factory and a pin was added to secure the hold-open. Where can I look for this pin, is it visible from the outside of the frame or it is in the inside? Thank You all.
Alf.