Ive edited your post. Swivel vice design on the 4.5” saws is pretty naff really. Especially with the tin foil gage they slap on the back to use with it. Don’t let that put you off them though they can be modified to be great saws if your up for doing the work. I don’t swivel mine now unless I need a one off obscure angle. I’ve made jigs that bolt to the saw base to cover all the common angles so repeatable cuts are always achieved.
In that pic there the arm is extended fully out so you cannot make the blade exposure any less even on a straight cut but you can retract it 2 inch for the wider stock. No it’s not been an issue to me personally. It’s well tensioned and cuts true. I’d prefer a moving bow like the smaller SIPs but at £75 total costs to build this it’s earned its keep over and over. That saw has made me more money than any other tool I’ve owned in the same time. It’s the only tool that is used on pretty much every job.
My biggest bug bear with it is it doesn’t accommodate short work. The stock needs to go past the half way point on the vice or I need to put something of the same size in the back side of the vice to make it hold firm. If I don’t the vice swivels and won’t hold it. I’ve plans to make a removable clamp to grip short stock but not got there yet.
Quick and easy way is to drill and tap the moving jaw at the end furthest from the blade. Put in a bolt that bears on the fixed jaw and adjust projection to match stock width.