ProTec . . . I have two different straight sopranos with fixed straight necks in ProTec cases, vintage and modern. I also have a MKVI clone from Taiwan that fits in a Protec.
If anything affects the fit, it would be the position of the bell pads and the design of the palm keys. Vintage sops, including MKVI and earlier Selmers have their bell pads in one location while modern sopranos have them positioned differently on the tube. But with the ProTecs that doesn't seem to matter with my horns. Same non-issue with the different palm keys. DAVE