I have an M77R .270Win flatbolt. It is in the 70-22xxx range.
In my discussions with a certain potato farmer he intimated that he believes there are less of them made than there are collectors wanting them, something like 20 or less of them made. Of course, since Ruger never posted the number made, we will just have to wait to see how many show up over the years. Congrats on finding one!
I lucked into the one I have. It showed up on a sale sight not listed as a flatbolt. I contacted the seller, asked for some pictures, and bought immediately for a very fair price. It was/is not pristine. I looks like it stood in a corner of a closet for years and the heel of the pad is a bit collapsed. Otherwise it's in nice condition. Seller said he scoped it, shot it, and put in the closet. I hunt with it some and shot a nice 3-point muley with it a couple of years ago. It's a consistent MOA rifle with most everything I've shot through it.
I believe that there were only about 7,000 or so long action flat bolts made with the 7Mag and .30-06 being the most common calibers. There are some .25-06s out there. 7x57 and .338Mag are extremely rare and I've never actually seen one of those.
Would love to see some pictures of the one you bought.