Came with one of my 527s. New in the retail package they came in. (I use Warne QD Rings on the 527s I have)
$40 Henderson
You can probably grab some Warne fixed 1B1Ms for ~$50 online which are a tad lower than these. 527 bolt lever and ocular bell diameter are always fun to figure out a combo that will work and play well together.
See PM for some eyeball measurements. If those might be a problem, please feel free to look at other options as I don't want you stuck with something that might not quite fit