AS per HP naming conventions all models XX50 series and XX55 series,eg (4350/4355,5150/5155,5550/5555,3950/3955 etc) are only because
1) 50series- Globally available models (including US)
2) 55series- availble in US only.
and because 55series has resco file browser.
But any one wants any info for file explorer see
http://www.vieka.com/ this is the explorer which overwrites windows explorer or pocket explorer.
what Pony99CA has said that only distinguish between 55XX series, but this does not implies for other series of iPAQs.