Agree could well be a 58 pattern - if so it will have: waterproof base, feather and down filling, rolls into an integral waterproof cover, toggles everywhere.
More than any other bit of kit, sleeping bags cause different opinions. One man's meat, etc, etc.
FWIW, I like the 58 pattern. I used one for 6 years in the TA and never once had a cold night, despite sleeping under a poncho in various bits of northeast Scotland. So I actually deliberately bought a new unissued one (curiously it is Dutch not British issue, but is otherwise identical to my old one). Have been out at minus 7 in a hammock, no problems at all.
Issued '58s are very cheap but usually are standard length (ie, too small - anyone over 5'7" or so needs a long), smell of ripe squaddie, and may have matted filling due to being stored compressed. But if you have a good one it is good kit. A modern bag may be a bit lighter and have smaller pack size, but the waterproof base is a bonus - with a good sized tarp and a little caution, you do not need a bivi bag.