Well found the item and to be honest - No idea what they are on about.
Would have guessed that all the filters are in effect restricted to 60mm. However all the filters shown have apparent different size filter material and all look well over 60mm.
Best I can arrive at and that is from looking at the ES site is that you have to cut out what is in effect an aperture mask and fit it then add the foam triangles to hold things in place, then go look at the sun.
And with all that I would go buy something else. Way too much of a DIY job and that could mean trouble. Ever tried to make a solar filter? I did, well tried. Final lesson I learnt was - go buy one that you simply screw on to the scope.
Also the ES site says SCT's, no mention of newtonians. Actually seems wrong for a newtonian as if I am correct and you reduce the aperture the secondary has a larger effect, most of the center is now secondary bits of metal.
Says OK for imaging, well imaging uses ND3 (1/1000) visual uses ND5 (1/100,000). So either 100x too dim for images or 100x too bright for eyes.
Will say an odd thing: As you appear to be UK, add to profile if you will, then go look at the FLO site. If they do not sell it then there is often a reason and you need to research a little. FLO is a good guide to what is "sensible", even what is "good".
I have never liked the idea of a solar filter on a newtonian, basically they are large aperture. Herschel wedges go up to around 100mm and yours is 4x the collection. OK sounds reasonable I suppose as it seems within the amount from a white light Herschel (no continium filter) but I would add a UV/IR cut filter in to the eyepiece.
Except for cost I would say go buy the ES/Bresser 80/640 refractor on their Nano mount, put a solar filter on that and go solar viewing.
But back to the question of field stop diameters, I can only guess that they expect you to cut out some form of aperture mask and so limit the aperture. And as that is DIY, I honestly suggest an email to FLO and ask them about the items they have and sell.