Thanks for the offer of help Jim.  The HC49/U case is marked:
"7372.8 TEW 7D".  They came from Jameco, part #14710.  When I
first checked to see if the middle pin is connected to the can
I thought it wasn't, but I just now saw that the can is covered
with a thin layer of clear insulation ... poking through the
insulation shows that the center lead is connected to the can,
so later in the day I'll give it a try with the center lead
grounded.

 - Nick -

James Paul wrote:
>
>  Nick,
>
>  Are you sure they aren't Crystal Filters for use in some sort
>  of communications equipment between stages?  ie IF Amplifiers.
>  Also, check and see if the center lead is electrically
>  connected to the can.  If it is, then yes by all means ground
>  it.  Where did you order them from?  Who is the manufacturer?
>  If I had the mfg and the part number, I could do some quick
>  research.
>
>                                          Regards,
>
>                                            Jim
>
> On Wed, 05 January 2000, Nick Taylor wrote:
>
> >
> > Another xtal question:  a few days ago I ordered a few crystals,
> > 7.3728MHz, 30pF, HC49/U case.  They arrived yesterday, and much
> > to my surprise they each have three leads!  I've never seen a
> > three wire xtal before.  As yet I've not had a chance to experiment
> > with them ... does anyone know anything about three wire xtals?
> > Should the center lead be grounded like a ceramic resonator?
> >
> > Thanks,
> >  - Nick -
>
> jim@jpes.com