A correction bit is very commonly used by reining and pleasure trainers as a transition bit to a longer shank and 'more' bridle as the horse gets more finished and switches to one handed riding. The bit shown has loose shanks and it can be ridden one or two handed.
The port on this bit is no where near where you have a bit to reach the roof of the horse's mouth.
Billy Allen bits are very 'dated'. The were used some by trainers 30 years ago, but there are much more effective bit nowadays. You won't find them in a trainer's tackroom much any more.
When you start looking for a bit to get a particular thing done on with a particular horse, you find out why trainers have 20 bridles or more hanging in their tackroom. There are bits that curve forward over a horse's tongue for those that do not like tongue pressure. You have high port for horses that like those. Dog bone training bits for horses coming out of a snaffle and on and on.
While I have 2 or more different bits set up, most of them have dust on them and I find 3 snaffles that are used on 90% of the horses that are in snaffle and about 2 or 3 shank bits for advanced horses. A correction bit is one of them.
How advanced is your horse and what exactly do you want to get done? What are you riding in new? All are relevant to what you want to move froward to.