.. although I am suspicious that the added brass jack plate surround is there to hide damage to the finish around the jack

https://thebassgallery.com/collections/bass/products/alembic-spoiler-4

I have seen those plates on a number of the older alembics over the years. I think it is there to cover/or minimise the extent of cracking around the jack socket.
same as this one.  http://usedmusicalinstruments.biz/vintage_alembic_4_string_electric_bass_guitar_custom_series_1_with_case_nr.html