I have several of the 107-FR model and one is like that. It feaked me out at first too. Here it is compared to one of my other ones:
I sent the pics to Arsenal along with an email, here is their response:
Dear Mr. <me>,
The manufacturing process in Bulgaria and our re-manufacturing process here in the United States will never allow any rifle to go out without barrel pin. Although at first glance it might seem that there is no pin installed, there definitely is a pin to hold the barrel in place. During the manufacturing process some barrel pins are ground flush with the trunnion block and painted over. This “conceals” the pin. I don’t doubt the existence of the pin, but if you wish to see it, you can remove the paint from trunnion block at the pin area, and the pin will be revealed (I do NOT recommend you to do this).
It is definitely safe to shoot this rifle. If you have any further questions, please do not hesitate to call or write us.
Sincerely,
Harry P.
Arsenal, Inc.
Las Vegas, NV
So I think you are fine. But you were wise to ask, can't be too safe. Enjoy it, I really like mine.