I answered this myself and because I'm nice I will share. George Benton interestingly called this the style for the talentless- because the way in which boxers are meant to go about their work in it means they don't have to be as reliant on super human reflexes.
Ideally you are meant to use it from range. The right hand protects the face from jab and left hooks. Due to the weight being proportioned further back then normal the right elbow will sit nicely on the side of the body - this blocks the left hook to the body.
The front shoulder does the job of the left fist in a conventional guard. It will block right hands aimed at the chin, though I get my lads to use their right glove to increase the size of the blocking area by placing it next to their shoulder. The turn of their body means that the right hook to the body is protected by the low left, or elbow and should their opponent loop the right hook any further round the lead elbow it should be called a foul by the ref (kidney punch).
Because the fighters weight being on the back foot the opponents uppercut is a non weapon too firstly because of the range the shot has to travel and secondly because the trajectory of the uppercut carries it upward and if the cross arm guy is standing right his body and head will be in line so there will be no target.
The oweness in this stance is on the boxer to maximise his movement - this happens for two reasons firstly the range he uses it at will allow him extra time to see the shots coming- secondly the weight being proportioned on the back leg will give him miles more movement. Try rolling with your weight forward!
Indeed the entire subliminal game of the Philly is the attraction of the low left. The opponent sees the gap and thinks he can bridge it- all the while coming into distance and walking onto punches - the opponent is set to move whilst the Philly boxer is set to punch.
The style has gaps but providing the executor of it is aware of these gaps he's no more in danger then anyone else. They secret is keeping it on the very edge of yours and your opponents reach. That way every time they commit you can catch them coming in or leaning forward - may weather will do this alot he will press- but only to a certain closeness to his opponent- once they get whose to it he often switches he tactics or guard to something else and he'll go back to the cross arm once they've forgot or he's had success with something else- then they have to chase the points they've lost again.
The openness to right hands over the top and right hooks - is not a problem. It's the mouse trap- you want them to attempt to throw those shots with the most obvious trajectory from the furthest away point- that way you can take advantage of the openness that comes from punching.
On the inside being side on cuts down your offence but the same set of defences apply with one or two changes. Firstly the uppercut becomes a danger as you lean in. However if you place your shoulder in the middle of your opponents chest you cut off their back hand.
The second difference is that instead of just sight- on the inside you use sight AND feel. You begin to pick up which hand is the threat simply by feeling it move to a ore punch position. That little move will tell you which side your on danger on- where they are now open. Only experience will allow you to second guess the shot but that's how a lot of great fighters demonstrate such sharp reflexes on the inside.