It is hard to tell from the photo. It may be a reproduction rather than an original.

Both. I think they also had a plastic buttplate in the latter years of production.

The aluminum buttplates should have the sides of the buttplate polished white. These were used on the featherweight rifles.

Scrollcutter is right. I have a 1961 M-70 varmet and it has a plastic plate that pretty much looks like the metal ones,except it doesn't have the point on the top. I am sure it is original,as I still remember when my father bought it new, and it has never had anything done to it(other than a lot of shooting).

Saw this on Ebay.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=73953&item=3697452179&rd=1
They seem to come up quite often. Are they steel or aluminum?

The one in the picture appears to be the aluminum one, I am 98% sure of that.. Notice the diamonds around the screw holes, the border and the outline and the checkering, although the picture is poor..It is IMO a low comb butt plate. and will have the teardrop on top, the plastic one does not and it has a different cut on the plate....

The steel buttplates have the same checkering pattern(diamonds around the bottom screw hole) as the aluminum plates.

Not to say that the one in the photo isn't aluminum. It probably is judging from the color.