[Heathkit] SB-104 Question

[Donald R Spoon]

I acquired a SB-104 tranceiver a couple of years back and have been 
enjoying it quite a bit.  I recently acquired an "E" board with the 
accessory CW filter installed and started looking into the process of 
installing it.  The manual I have is actually for a SB-104A, which 
hasn't been  a problem until now. 

The manual shows a 400 Hz push switch in the bank nearest the Microphone 
connector.  According to the manual its purpose is to switch in the 
narrow CW filter when doing CW.  My tranceiver is different in that it 
has a switch labeled  100 Hz, and pressing it turns off the lowest digit 
on the freq. readout.  This behavior isn't mentioned anywhere in the 
manual I have.  

I have NOT plugged in the new board with the additional filter as of yet 
for the fear that my box might not be an "A" model and may not accept 
the new board.  I don't have any manuals or paper work that might help 
beyond my SB-104A manual.  When I got the SB-104 originally, I had to 
reverse some "golden screwdriver" mods that had been made. 

Can anybody here explain the difference between my box and the SB-104A 
and/or reassure me that I can proceed with installing of the new board?

Cheers and TIA,
-Don Spoon-
K0APK