On Tue, 24 Dec 2013 18:30:08 -0500, Stormin Mormon
wrote:

One
half of the threshold is loose, and needs maybe
four or so anchors.

I read the replies. So this is a repair, right?

Remove the screws and have a peek under the TH. Chances there could
be some old silicone caulk and plastic anchors in the holes already
drill in the concrete. Clean all the off well.

Put new anchors in, run a couple of beads of silicone caulk (draft and
water intrusions prevention under the TH).

Buy screws which are the same as present used on the TH being
repaired.

My take is this is not a new install....