Can you pull the grill cover off it? Is the cabinet sealed so that the only access from the front? Judging from the photo, I would wager it's a 115M or 115V from the early 1970s. See photo of my 115V below.
The only access to the inside is from the front; the back is sealed. Mine only has one jack; later ones had two. The Sonic cabs open up from the back, I believe.