matching cars to industries

[Go Big1]

I am going to a train show/swap meet this Sunday, and plan to hunt around for some bargain hoppers. My problem is that I don't really know what I should be looking for as it relates to my layout. I have a Glacier gravel plant that I will locate on the layout, and am not really sure what kinds of hoppers would have been brought in and out of this plant. Time frame is 70's or so. Would these hoppers be covered or uncovered. I saw a really good deal on some 34' covered hoppers online (see pic below), that would work great (as I want to predominantly model Rock Island), but I don't know if these are the types of hoppers used to transport gravel.

So, can anyone tell me what hoppers were used in the 70's to transport gravel?

Thanks.