There are at least 3 different covers used when running horizontal....
The early style phenolic, not shown here, to be used on the machines (early FP2) where the vertical head is removed for running horizontal.
2. Later manual machine cover.(hard plastic)..This is for "square" head machines (FP3) where the vertical head is not removed , but rather slid back.
Shown here:
Believe this is the cover style you are looking for....Need deeper trough at the rear to clear the vertical head.
Third type is used with the NC machines (non flip)
Softer more flexible plastic, has foam seals to keep the dirt out and lube oil in.
Tried this cover on my FP3 and it won't fit up because the rear trough is shallower than the earlier version cover....
Believe your best chance to find the correct one would be to contact Singer...this stuff is just too old for DMG to know anything about.....
Cheers Ross