Sac for Faber Castell Osmia 223 ?

[Pterodactylus]

Hi All,

My Faber Castell Osmia 223 needs a new sac.

Faber Castell Osmia 223 - F by Ptero Ptero, auf Flickr

Faber Castell Osmia 223 - F


Currently it had a straight sac installed.

Peg width is 6mm so I would choose a size 15 as replacement.

But the filling volume of this push button filler was always quite poor so I thought wouldn´t it be an option (or even better) to install a necked or necked and tapered sac as replacement?
As I read often that Necked sacs are better for push button fillers.

Pendragons offers the following sacs (beside the straight ones):

• Size 15 x 1 7/8 Necked
• Size 15 x 2 1/8 Necked
• Size 15 x 2 1/4 Necked and Tapered

Can you please advise me what size to choose and if you also would try a necked one instead of a straight one?

And is my assumption correct that also for necked sacs the size stands for the narrow part (just like for the straight ones)?

And what exactly is the second number?
Does it mean that it widens e.g. for (1 7/8)/64 of an inch?
How is this for necked and tapered ones?

Thanks