Good observations and questions.
Security Companies - the larger ones like Pinkertons, Burns, Brinks, Wells Fargo, etc. all had their own company specific designed badges and each one was/is serially numbered and assigned/issued to a specific guard. When filling out reports the guard would sign them and include his badge number. You can google those company names and see examples of their badges.
The more "generic" badges seen today are usually a silver shield with a large star inside and some sort of title like "Security Officer", "Special Police", etc. They do not have a company name or a badge number on them and are mass produced and offered by various uniform companies as accessories to their uniforms.
The photo of the Giant Security badge containing the great seal of Maryland is somewhat of a puzzle to me. All law enforcement agencies in Maryland have that seal on their badges, but seeing it on a badge issued by a private company makes me wonder. It would imply that this company is part of the state's law enforcement or that it is officially sanctioned and sponsored by the state in some way.
This is in direct opposition to my own experience and knowledge of Security Services companies. I know that some states have laws which regulate what security guards can and cannot wear in regard to uniforms and badges. For instance in some places, shoulder patches have to be red for security guards and blue or black for police. Use of any state insignia is more often specifically restricted to police than required for security guards.
Uniforms also had to be distinctly different from State and Local police. Security Guards have specific duties and normally do NOT have the power to arrest or detain persons unless they are deputized by law enforcement. Use of the official state seal on a security guard's badge might imply such deputization.
The object in the pocket of the guard in the photo appears to be either a small notebook or a handbook issued to security guards. As a Security Guard with a nationally known agency in 1971-72, I had such a handbook issued to me and was required to carry it on my person at all times. The book contained general rules, regulations, and instructions and had sections on wearing of the uniform, safety, first aid, and firearms. I recall that the back of the book had spaces to write in the names and phone numbers of supervisors, home office, and client security managers. We generally carried those little books in the shirt breast pocket.