First of all, your pictures are too big for me to view without downloading them to my computer and using Windows picture viewer to scale them down to my screen size.
The number on the back of that speaker, 81 0145 is the Sunn part number for a 15 inch woofer.
Check this thread:
https://forum.sunnstillshines.online/index.php?topic=754.msg16586#msg16586 My guess is that what you have is called "Coliseum Monitor". Not much in the way of pictures in any catalog I have, but the dimensions are supposed to be 21"x25"x29".
I found two different references to a Coliseum Monitor:
1971 "Feel the Sound of the 70's" Sound System catalog
Coliseum Monitor: One 12" Sunn PA transducer, One high frequency horn with Vega Driver.
Two cabinets lock together face to face for transport, 18" high, 20" wide, 17.5" deep, 50 lbs.
1979 catalog
Coliseum Monitor 100W 8 Ohm
1-15" 81-0145
1-50W Driver/Horn 81-0294
1-Tweeter 81-5150
Dimensions HWD 21"x25"x29" 51lbs
Not sure about the 81-1300 on the back of the horn driver. Might be a later replacement for the 81-0294 or it could be a different Monitor entirely.
