Speedmaster Part: 321-1794 Hour Hammer Spring

My watchmaker sent me a note:

"
The number is 321-1794 - Hour Hammer SpringI received the 861-1794 listed in some places as interchangeable but it is too short."



He cannot source the correct 321-1794 Hour Hammer Spring.

Can anyone point me in the right direction or have one that I can buy?

Thanks,

Tom