...No way is the bit you have pictured, a Tom Thumb, BSMS . IT has sleeves, there is some curve to the shanks, plus a non protruding , non pinching joint...
It is extremely close to being straight, the proportions are the same, and the sleeve functions just like the regular cheap TT, which does NOT allow the top to swivel into the horse's mouth - unlike what some have written about them. It is, structurally, every bit a "Tom Thumb".
I once owned a cheap Tom Thumb. I'm not crazy about cheap bits and have no great desire to go buy a cheap Tom Thumb to prove my horses would ride they same...but they did.
The knuckle in the top picture is no more extreme than the knuckle on my O-ring bits.
"The bit you have, I simply refer to as a jointed mouth curb, and while TT also are jointed mouth curbs, all jointed mouth curbs are not TT!
No kidding. You can call it anything you want, but it clearly is functionally identical. It is just that Tom Thumb bits do not work the way a lot of people, including Mark Rashid, claim they do. And that is obvious if you simply stand next to the horse and watch what happens when the reins are used.
They are not scary bits. Cheap ones are often used by people who themselves are scary on a horse, and that causes problems. People who have no business using a curb bit will buy them to impose control on a horse, and that gives them bad reputations. But they work just fine when ridden using with frequent slack in the reins, timely release, and on a horse who has been taught how to respond - which is true of a huge number of bits. Not every horse will respond well to them, just as some horses prefer a double joint snaffle to a single joint snaffle, and some prefer a curb to a snaffle or the reverse.
How much bend there should be to the sides depends on how the horse carries its head. A horse like Bandit, who often carries his head close to the vertical, will find a TT reasonably balanced. A horse like Mia, who typically carried her head at 45 degrees, will not find them balanced. But then, a strongly curved shank that balanced well for Mia does not balance well for Bandit. Bandit will accept Mia's old favorite, but it doesn't function as well for him as the TT because the shanks have too much curve for how he typically rides. It isn't balanced right for Bandit, although it was perfect for Mia.
Ideally, at rest, the weight of the reins should have the far end of the shank resting under the mouthpiece. With Mia, whose head was normally at 45 degrees, the weight of the reins would rotate straight shanks 45 degrees to rest underneath - and that removed the period of free rotation prior to the curb strap tightening. Thus any additional pressure on the reins would cause immediate pressure in her mouth, without any warning (signal).
But Bandit carries his head steeper, and the TT bit stays stable with the ends supported. Thus there is still 45 degrees of rotation available before the curb strap tightens and pressure is applied to the mouth.
The Argentine bit pictured:
is functionally almost identical to a TT. It has a little more curve to the shanks than a TT, so it will accommodate a horse whose head is tilted out more...slightly more. But I used to own one like that, and used with Mia, it would also rotate by the weight of the slack reins until the curb strap had tightened - so it gave no warning either.
All of this is apparent if one just takes these bits out and watches them while standing next to the horse, using horses who naturally carry their heads at different angles.
BTW - Bandit seems reassured by constant contact when he speeds up, so we've been using snaffles exclusively for the last 6 months. But I could pull the TT out of the closet tomorrow and he'd be fine with it - better than he would with the Billy Allen.