Screw type handcuffs

Hello fellow collectors, I made a cool find at a gun show this week, I have seen pics of these cuffs before but never a name for them, I know one of the experts has an idea. They both are marked FM.SW I know this is not useful, but there the only markings. They have a small inner cylnder that is threaded standard and can be locked by inserting a flat blade screw driver. I have never seen a key pictured with these and I do not have either. There is a small hole on each cuff to attach to a gang chain or smaller. They both operate fine. My feelings are early 1800s but would love to here some info from the experts .
Sincerely,
Gav

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What you have is called a Plug Cuff. Normally, there is a small plug that would be screwed into the open hole, where the key goes, once the cuff is locked on. The plug prevents using the keyhole to unlock as well as tampering or debris getting into the locking mechanism. FmSW could lead to the maker of these cuffs. Joe Lauher is our go to British cuff guy. My thoughts are that it may be Firman and Sons or maybe  Fitzwilliam and sons? Though Fitzwilliam and Sons is generally marked with FWSM. So, i cant say for sure who would have made them. The pic im sharing is of a Nichols, plug 8 leg iron in my collection. The key shown is the original. The key is threaded into the first part of the locking tube, once clear of the first set of threads, it will drop into the tube a bit and contact a second set of threads that is reverse of the first set. On mine, you turn to the left to enter the keyway, the key drops in and you turn to the right to open the cuffs. Reverse order to lock them and remove the key. The two prongs on the top of the key are used to remove the plug from the locking tube. Most times when these were used in a chain gang, a large ring, connected to a chain, would be slipped over the cuff and the cuff locked on the prisoner. That chain would connect to the cuff on the next prisoner and so on. Most plug cuffs generally date to around late 1800s. Nice find!
-Richard Knip-

the key turns a castellated nut in the tube to unscrew from the threaded stud on the bow of the cuff.

Thank you for sharing from your collection and all the information. Happy Cuff hunting.

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Richard is correct.  They are Fitzwilliam and Sons.  FW SM. 
If you look closely at the cuffs you can see the logo.

Very nice find. Congrats.

I'm surprised no one has mentioned it but the cuff have been taken apart.  The squarish holes on each cuff was where the chain was attached.  You can see from the picture of the leg irons above, that the tabs have been removed.

Ron

An interestingly the hinge rivets are raised? Been replaced.?