After market Grass Bucket fixture
I have a Suffolk grass box which is fitted with a strengthening bracket, it looks like an after market product, there are no rust holes to cover so I am guessing was an identified weak point.
I would say home or dealer made ; although the apparent part number in your image may suggest otherwise. Needs must when faced with a rusted or split grass box. The advent of “ plastic” boxes on larger machines - 20” upward - that were not properly supported or suffered from point loading on their brackets gave rise to some quite professional looking repair solutions. If you look at the prices of many replacement boxes, both steel and plastic it will become apparent while time and ingenuity was worthwhile in effecting repairs. In 1993 the retail price of an Atco 24 alloy box was £196 and the last time I bought a steel box for a 20” Ransomes Marquis it was nearly £300.
Having found out what worked for various boxes we made and kept some paper templates for speedy production of alloy repair patches and reinforcements.