It seems to be that Heuer "repurposed" some of the bridges in stock to fit whatever suited their needs at the time. If you search for "pits" on the forum, it should turn up a few threads where we discuss these bridges that have had the original number removed and a new number stamped in the resultant pit. If it's an 11 being restamped as a 12, then usually the bridge is nickel-plated rather than gold. Maybe this one was originally a cal 14? We see that later in the 70s Heuer seems to discover a stash of nickel-plated bridges - it's not uncommon around '76/7 to see these with a 12 stamped in a pit.