Yep, arsenal 26 with a s/n in the 10 million range. That makes it a 1966 production.
It is indeed the 16" barrel model, commonly called the "paratrooper" model. Not all of your photos are showing up for me, so I can't tell how that scope is mounted. There was a 16" variant called the "cowboy companion" that had a scope mount on the left side of the receiver.
However, they were not originally produced that way. This was a commercial adaptation (done in China on surplus military weapons) before export to the West. The bayonet lug has been cut off as well, which was common on these.
If the numbers are matching on all the parts, it's very appealing to collectors. The scope actually hurts the collector value, especially if the receiver cover has been altered or replaced to accommodate it.