I have seen this before. I forget where. Note the young man has his pants rolled up to display the fact that he has two prosthetic limbs. The braces are part of the limbs stabilizing the thigh corset to the leg portion. Both legs are amputated below the knee.
This was at a period in time when pool/billiards was most popular and this image was to illustrate it was possible for him to play this quite popular game despite his disadvantage. (A public doubt existed towards people with handicaps for quite sometime). This illustration may have been part of an advertisement with the details of the table not significant based on the real message.
Prosthetics have come a long way, not only mechanically and technically but additionally, public perception and acceptance.