Ok, this is just my opinion, and I make no claims at being an expert. However, it would seem to me, that one would be most likely to find pira in pictures taken prior to 1920 more closely to the turn of the century for a few reasons. One after the American ban on traditional weaponry (Im forgetting the exact date), I would imagine the incidence of people wearing traditional weaponry, at least in front of American cameras, would have declined. Secondly, part of the reason I am more inclined to think pics with people wearing pira would be closer to the turn of the century, is that it is at this point, before the US administration started to tighten its grip in the region, that relations amongst US/Moros was most friendly, with few incidence of attacks against Americans reported, so this would be the time when it would be the most safe to take pics of armed persons. Finally, a pic taken by Maude Jenks, (I think it was taken by her, but I cant remember for sure) of a couple of warriors wearing pira was around the turn of the century, and is of striking similarity in style of picture to the type taken (not just the dress of the warrior, but also the picture style itself, the grainy BW, etc...). There is also a limited number of picture studios, and photographers who did this kind of subject at a time, and they were most popular in the early turn of the century. The following site/database is nice as it often contains the date of the pic and the studio name/photographer http://webcat.library.wisc.edu:3200/SEAiT/
While we most associate pira with Yakan, at least as far as Cato's book is concerned he does say the pira was found throughout the Sulu sultanate. However, he distinguishes the ones as Yakan as the ones featuring the typical Yakan features of abbreviated punto, and cockatua style.
Anyways, lovely pira.