The Doctor, seeing his client more attentive than alarmed, was greatly surprised. He must be matriculated, said he to himself— ' A h ! ah!' added he aloud; 'you have been obliged to shave off the lock. You have been prudent; however you need not have done so, when putting yourself under my hands. The case is serious; but you don't know what I have courage to do in a time of need.' To understand this mistake of the Doctor's, it must be known, that at that time, bravoes by profession, and villains of every kind, used to wear a long lock of hair, which they drew over the face like a visor on meeting any one.
This appears to be a case of mistranslation from the Italian. The quote is from The Betrothed (I Promessi Sposi) by Alessandro Manzoni. In the Project Gutenberg edition, this passage reads as follows:
The doctor, perceiving his new client more attentive than dismayed, marvelled greatly. “He must be enrolled as one of the bravoes,” said he to himself; “Ah! ah!” exclaimed he, addressing Renzo, “you have shaved off the long lock! Well, well, it was prudent; but placing yourself in my hands, you need not have done so. The case is a serious one—you can have no idea how much resolution is required to conduct these matters wisely.”
To understand this mistake of the doctor's, it should be known, that the bravoes by profession used to wear a long lock of hair, which they pulled over the face as a mask in enterprises that required prudence as well as strength.
I expect neither matriculated nor enrolled conveys the real Italian meaning well. My guess is that the original means he has taken part in some ceremony in which you join the bravos. It would be good to have somebody who knows Italian look at the original.
Finally, since Project Gutenberg offers for free a translation that appears from this passage to be much better than the one you are reading, you might want to consider reading the Project Gutenberg translation instead.