The item you called a covering ring, I usually call that a bash guard.  Don't discard it, if the chainglider does not work out you might want to use the bash guard again.

I'd agree with geocycle that it looks like a Raven Sports Tour, I think that's the only Raven with the brakes on the back of the fork, but it was possibly always that colour. Thorn included a stealth black version for a while, I don't thing I've seen one (maybe because it is that stealth) but there was one advertised here a couple of years ago:
https://thorncyclesforum.co.uk/index.php?topic=14974.msg112886#msg112886

Looks like a Raven Sport Tour, probably 537L. The colour and lack of stickers suggest a respray.

...
...  I am still deeply suspicous of the performance losses associated with "one-bye" (single chainring) drivetrains in all but the middle cogs of the cassette. The variations between smallest and largest cogs on the cassette occur at the most extreme chainline angles and the difference in cog diameters is also extreme (i.e. 10t vs 40t+). Perhaps sheer convenience in shifting and relatively greater simplicity accounts for their popularity. I'm not yet sold, preferring to run my derailleur triples (three chainrings, front mech) and shift to keep my chainlines as straight as possible to minimize friction while maximizing component life. ...

Fully agree.

My derailleur bikes all have triples, except my road bike is a double because I bought that as a complete bike, not one I built up from parts. 

My two derailleur touring bikes, the Thorn Sherpa (26 inch wheels) and Lynskey Backroad (700c wheels), both have 3X8 half step plus granny drive trains.  When I avoid the two most cross chained gears for each chainring, I still have 18 effective gears with a total range of 558 percent.   I went with this gearing for touring, as it gives me a few widely spaced gears for hill climbing on my granny chainring and closer spaced gears for where I spend most of my time on flatter ground.
https://gear-calculator.com/?GR=DERS&KB=24,42,46&RZ=11,12,14,16,18,21,26,32&UF=2204&TF=72&SL=2.2&UN=MPH&DV=teeth

The technology is 30 years old, but it beats the 1X drivetrains in performance, plus the cost of expendable parts is much less.

That said, I understand the preference for a 1X system for a single shifter with sequential shifting, anyone that rides a bike with a Rohloff certainly can appreciate that.

In the common game of comparing derailleur efficiencies with those of hub gearboxes, there's a bedeviling factor: The derailleur transmission in a real life comparison on the roads is not statically efficient: it declines dynamically from perfectly clean and tuned at the outset of the test as the chain and gears pick up dirt.

That also happens with an uncovered hub gear transmission. But more slowly, as the derailleur hangs lower and closer to the ground. And with only one chainring and sprocket the hub gear system is easier to clean than a derailleur system.

Brakes also make a difference on an uncovered transmission, a drum, roller or disk brake doesn't spray water and muck around like calliper brake does. Worse brake I had in this respect was a U-brake, placed under the chain stays, ideally placed to put the maximum amount of water and muck onto the chain.