Giving this some further thought (for for what it’s worth) I would think that this metal 3 sided bridge would have been found in all of the 12 1/2 sets. And even perhaps the earliest sets with the full tray. The key need for it may have been shipping.The part itself would be superfluous to a kid who got one of these sets. It has no function in building any model. But the MN based middle tray in these sets can sometimes slip off the support at either end of the case, as it is not a tight fit. Having that tray slip and fall onto the lower cardboard insert and parts on same could cause some damage. The bridge would avoid that potential in shipping out the sets. The set I mentioned had that bridge, but an original silver lower insert as well in excellent shape. That says to me either it was lightly used, or that a very meticulous kid owned it and would carefully put it back together (some kid we’d call obsessive compulsive today, but then those kids are the source of some great survivors.) But most kids would remove the parts from the MN tray and take it apart, never to be reassembled as shipped again until one of us old guys got hold of it 60 or 70 years later. Then that bridge would have been pulled out to get the parts off the cardboard insert below. It might then have sat around in the case. ut just as likely went to the side with the cardboard. So finding a set with the bridge in place might be unusual today, but I would guess that all these sets were shipped with one of the in there to hold the MN tray in place.