Yes, I know the "problem" with the old type gps like 276C (I own one myself)...
I unlocked the "conventional" way with Jetmouse 1.5 Fix for the id of the 276C and it worked fine for me.
You can also try with the unlocker here [Only registered and activated users can see links. ]
Test with a small part to save time, it takes some time to transfer to the hopelessly small and expensive memory cards used in theese gps.