I just checked my barrel markings. They are marked the same as yours, that being 17.6 and 17.8. There is no bore size stamping on them, same as yours. I am assuming my bores are around .729 which would make the chokes full and improved modified. They throw tight patterns. You are under the assumption that your bores might be 18.5 because you saw another gun like that. That could be wrong. You will only know by having the bores measured, or better yet, by patterning it.
My 16 gauge is marked 16.2 and 16.4, full and modified.
Are you sure the pic of the barrels you show as marked 18.35 is actually a Brno shotgun? Counting my 3 guns, the one you are looking at and a couple of others I have seen, they all only had the choke constriction stamped. The actual bore dimensions were not stamped on any of them. I have also not seen that little circular mark with a line through it, beside the bore diameter in your pic, stamped in any of the Brno guns I looked at. Of course I haven't seen them all.
If you don't want to open up the chokes and can't measure bores or try the gun before hand, I would be cautious because I think you will find it is full and modified( if not improved modified) which will be something you stated you don't want.
yes you are right one i am going to buy does not have that symbol(it is phi symbol for diameter), neither it has 18.35 marked.Actually that pic is of also brno sxs i took from web just to show the bore dia.here is the link
http://www.gunsite.co.za/forums/showthread.php?29539-Got-this-Cz-brnoactually i want it to be at least one barrel modified (1/2) and no matter other is imp modified or full because for second shot bird will have gone away.
