Yes, the insert you speak of uses pins which I think go in the receiver and pivot pin holes IIRC? I haven't used this insert since I have the lower insert and clamshell.
The regular lower insert is nice cause you can access the inside of the receiver, upside up or upside down, for uses other than barrel installation (I used it to hold my lower while I cranked off the Colt staked telestock castle nut). I am assuming this is white delrin insert mentioned - the one that goes in the magwell (not the ones with pins). Very handy, but I would use the clamshell for barrel work. They come in black too I think. There is someone on eBay or Gunbroker or such that is selling solid aluminum lower receiver inserts, supposedly much tougher and coated to prevent scratches. They list it as "Receiver Block from Hell" or something like that. I want one!