I need a part that will act as a spacer but it needs to be threaded on the
inside to take a 1/4" bolt on both ends. It needs to be about 1 5/8 in
length (no need to be exact).

I searched in Lowes and HD and couldn't find a match. The have threaded
rods in varying lengths and sizes but they don't have the "female" version
of this. I tried to search online but cannot find it since I don't know
what it's call.

Application - I need to attach a custom mail box to a metal door (hollow in
middle about 1 5/8 space). There are 1/4" bolts from inside and outside (I
don't want to use nuts on one side), I found some hexagonal shaped nut
couplers but those are too big diameter wise, I am trying to find some that
will not require me to drill a hole bigger than the head of the bolts
themselves. I remember seeing these parts used in door locks, ceiling fan
rod extensions etc...

Thanks,

MC