It's the exact same display module as in the 53131A/53132A. It's a Samsung INB-12MM53T, Keysight part number 5040-0023. The VFD part itself is an Itron CC1187G. I've also found a possible part number for the VFD as an Itron CP3033A (although that might be the VFD for the 34401A, which would not be compatible with the 58503A). A brief search doesn't turn up any sources for the VFD, much less the entire module. Alternatively, if you found a 53131A or 53132A with a bright display at a reasonable price, it could be used as a parts donor. Stan