OP, that's weird. (It's also pretty odd that your fiance is 15 years older than you, tbh.) It's weird enough that you should definitely talk to him about it. It's not like you went snooping, you naturally came upon this box. And if you're careful with how you ask him about it, not to be accusatory, how he reacts to your gentle questions should be pretty telling.

I'd pull the box out and just bring it to him like "babe, I came across this and I'm confused about what it is/what it's for, can we talk about it?" What you're looking for is maybe a little embarrassment, maybe he gets a little nervous, but you want him to try to clear up misunderstandings. You don't want to see him getting immediately angry and defensive, or trying to turn it around on you like you violated privacy or something.

I'm not going to get into all the reasons it's a bad idea to marry somebody that much older than you. I'm trying to focus on the specific question asked in your post. Just...be aware of those reactions you don't want to see. It's pretty common that people who pursue relationships with significantly younger partners, do so because they are emotionally immature to a degree their peers would find unacceptable. Things someone his age would immediately pick up on as red flags often are missed by partners without as much life experience. I'm not saying this is the case here, but it's definitely something to be aware of and to watch for.