It's a custom cover, a bit too thick. 1/2" ply, should be 3/8". Simple to make and fasten.
A late post on this.
My AR is just like that one, with a wood enclosure as well.
Original as far as I know.
If it is not factory, then someone offered wood enclosures for it back in the day.
I picked mine up at an estate sale a few years ago.
It has a bit of a hot spot (darker wood) marking on the top if I recall.
I will need to un-cover it from its storeage space and take a photo or two to show you.
The sad story is they also had 1 AR-LST speaker with it!
The AR amp was sitting on the speaker when I grabbed it.
Thinking on it the lady at the estate sale said the other speaker maybe in the garage, do you want that too?
I passed on it or them.
That was before I know about AR speakers.
I may have been able to get them as well, for a song.
I doubt anyone else THERE was interested in the speaker/s.
They are huge and the house was on a steep slope.
It would have been a work out getting them out of the house and on to a truck,
cause a car wasn't going to do it.
And to top it off I had driven my Silverado that day!