T550- Can't remove Base Cover assembly

I received my Thinkpad T550 this past Monday and wanted to familiarize myself with it's innards as I intend to amend/replace some of it's components with new ones at some point. (SSDs primarily)

I followed the Service training videos I found linked on another thread in this subreddit, for somebody with a similar question, as well as in the sidebar, covering the internal hard drive for the T550:

https://www.lenovoservicetraining.com/showcase?sid=589&key=TGVuZw==

I cannot get beyond the "Lift the base cover assembly from the system" step. It doesn't come off. All the sides are snapped free, all the screws are loose, but it refuses to come off. It appears there are three protruding metal holders which fit into holes in the case which are still holding it down. I took a picture of one of them. (I had snapped the system back together at that point). I tried moving the case laterally to the rear of the system but it refused to move. I was going to force it but it required enough effort that I was afraid I would snap the case long before it cleared the metal holding piece. It's not entirely clear what special action I need to perform when performing the "Lift the base cover assembly from the system" step to get past these. I took a picture of the metal tabs I'm referring to:

http://i.imgur.com/ATztsAa.jpg

I'm sure I must be doing something wrong here but I can't figure out what it is.