Swan pen take a number of different size sacs, the majority of these sacs are the Necked variety.

There are a few models that take the Streight 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

to enable correct fitting, once you have re fitted the section pass a rod up through the section and inside the sac to ensure it is not twisted.

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.

Parker '51' MK III Aerometric Standard Size Connector