A modern similar product from the original supplier, Edwards Signalling.
https://www.edwards-signals.com/file...talog_Page.pdf
It is rated for 6VDC operation as well as AC. (Sorry, no 12 V seen on their website - an argument against converting trucks to 12V?) Model 725 are on ebay as low as $9US.
On edit: it looks as if at least two different buzzers were used.