whalerman said:
If it's been cut down it is. If it's original, it's a 5.5" barrel.
wnycollector said:
I carry my 1924 S&W M&P 5" .38 in a $5 flap holster I picked up at a gun show.
That's good for you, but finding a 5" holster for an M&P K-frame is not hard. It was a terribly common length up until about the mid 1950s. I've got a few holster for that length/gun that cost beans sitting around. They run about $7 on Ebay.
But the OP is talking about a 5 1/2" N frame. That's different than an M&P - quite a bit different. Unfortunately, the 5.5" N frame barrel was pretty unique to the 1917. You have to either get:
a) WWI military holster for it. Rare and pricey.
b) later service holster for it. Some were made, but still rare. (see below)
c) get a custom made from El Paso or similar maker.
There just aren't any modern 5.5" N frames or holsters for them. You'll have to dig up an old one or have one made. I felt fortunate to find this one for mine at a gun show, an older service type made by one of the WWI makers but true to the military style with the swivel and flap. The swivel is awesome if you ride a horse! I've also got an original M1912 swivel holster I carry my 1911 style guns in. These are great holsters if you can track one down.
You'll have to dig around at gun shows, or order one from a reputable maker.
W.H. McMonies holster for a 5.5" 1917 (brass swivel hidden behind flap in photo):
This is a 5" M&P - a whole different and much smaller gun:
Colt and an M1912 swivel holster: