I think the metal was a new thing, but realized after all of the fraying harness complaints that it was NOT a good idea! Oh wait, you have a 3/09 that has metal?? I would call and ask about it. And the other one with metal as well!
I would at least call - if your harness is new, I would just ask for the new tongs. .
The buckle tongues aren't available by themselves, they come attached to the harness.
Ignoring the FPSVD, here's the history on the Britax buckles and why some are plastic, then metal, and now back to plastic...
Original plastic: Britax buckles used to be single click - with most seats (and Britax now as well,) you get a click after each tongue is inserted. With the single click buckle the positive click only happened after the 2nd tongue was inserted. Some people complained a lot about it, so they changed to a double click buckle.
Metal buckle: When they switched to the double click buckle, all the tongues had metal in them. I'm sure a search here would show the exact date that happened, but it was early 2008.
New plastic buckle: For Canadian seats the change happened sometime between the end of February to the middle of March 09. (I don't remember exactly now, but I had a whole bunch of seats at work that I used to get a 2wk time frame.
) My guess is it probably changed on US seats right around the same time.
A lot of people have had fraying problems with the metal tongues, and I think that was an unanticipated problem. So they just started encasing the metal tongues with plastic again to prevent the fraying. And there you have the history of the different buckles.