Maybe others have had issues coming back with cameras, electronics, optics, etc. but I've never once been asked to provide proof of ownership (as in I was in possession of it prior to leaving the US). I've been traveling internationally for 2 decades now - business and pleasure - and the only time anyone domestically even remotely expressed interest was in NY when I returned from a Canadian moose hunt. When I first started traveling with guns through RSA, I obtained a 4457 for everything - firearms, scopes, archery equipment, optics, cameras. Aside from the firearms, most of these things have changed frequently. Since then, I've redone the firearm 4457's to have only the firearm and I've trashed all the other forms. Even after traveling to the Far East for several years, where the opportunity to by cheap electronics and cameras exists, no one questioned what I carried back.
Since my focus for the past few years has been on Zim, my upcoming trip is routed thru Dubai to completely bypass RSA and the BS at O.R. Tambo. If headed to RSA, or transient thru RSA is unavoidable, use someone like Henry or AfricaSky Quest House to get your SAPS520 pre-approved (which will have a notarized copy of your 4457).
The CBP4457 is good for life if nothing changes, and in my experience, the only practical use it serves for the international hunter is for foreign counties with more stringent firearm registration policies to claim that you, the foreign visitor, have complied with some form of "rigorous" registration.