Assuming your horn has the bell keys on the LH side (as you're behind it) no, it will NOT fit in that case. As someone who plays old Selmer Super/Radio Improved alto and tenor, I too have struggled to find a gig bag that would fit those horns until I got the Protec Pro Pac Contoured (NOT the "Max") case. Having said that and to my complete dismay, over the years, Protec has changed the padding inside their cases and (as in the "Max") your horn having the bell keys on the LH side, again......it won't fit in *SOME* of the Protec Pro Pac cases.
For my Selmer Super, I use the Protec Pro Pac "XL" contoured case and it fits very well. I also have some 10+ yr old Protec Pro Pac contoured cases (the "regular" models) that it fits in nicely, but the newer, non "Max" models are a crap shoot as to fit.
I assume this is a picture of the interior of the case you have?:
You can see the interior is designed to fit "modern" horns with the bell keys on the RH side. Could you cut away at the padding inside to allow the LH bell keys to fit? Sure, but I sure as hell wouldn't do it. There's no guarantee you'll get it to fit AND with all due respect, it would probably look like crap when you got finished hacking away at it. Unfortunately, I'd advise to suck it up and send back or try to sell the case to someone who has a "modern" horn it'll fit and then look around for another case. Sorry. That's the best advice I can give ya...
*EDIT* THIS older Protec Pro Pac contoured case WOULD most definitely fit your horn:
https://www.ebay.com/itm/Protec-Con...AOSwI-heEOGz:sc:FedExHomeDelivery!54401!US!-1
See the difference in the interior? Horns with LH bell keys will most definitely fit in this case. Again, the newer Protec Pro Pac contoured cases.....maybe, but there's much more padding on the interior like your Max case.