Swan pens take a number of different size sacs, the majority of these sacs are the Necked variety.
There are a few models that take the Straight Sacs but very few.
The sac in Swan Pens are meant to fill the barrel Diameter as much as possible so they will be a tight fit.
To ensure you have the sac fitted correctly you will have to remove the Nib and Feed from the section.
Fit the new sac to the sac peg and refit the section back into the barrel with the new sac on, pass a rod up through the section and inside the sac to ensure the new sac is not twisted and is fully open. Invert the rod when re fit the sac to ensure the sac enters the barrel fully.
Only then refit the nib and feed.
As a rule of thumb if you have a model without a number, but the size is the same as a numbered pen the sac size will be the same in most cases.
The sac Size Chart is detail from the smallest to largest sac size.
At the end of the chart are some Blackbird and a Swallow Models.