The 1987 Silver Jubilee 50W head (switchable to 25W).
Marks that can be entered for this maker: Serial number (plate), Transformer code, Speaker code, Born-on / QA sticker, Panel features (no code).
Nothing predates 1987; components only push the date later.
Sold-price aggregates are not collected yet, so the archive gives no figure — condition, originality of the transformers and a documented recap move this model more than any list price.
Evidence: Blue Book of Guitar Values — marshall