Yes that is the clip slot. and Yes I believe you have a NM Model 70.
Which by the way was quite popular back then for high power shooting. They were AND still are great guns.
I have one just like it in 308 Win.
Suckers are dern accurate and will serve you well if you decide to get into target shooting.
Get some stripper clips and learn to load it, they are fun to shoot and fairly fast.
