I wonder what the string around the safety valve casing was for?

Peter Kerslake says "Sorry , but I cannot help about the string, but at this date little 1421 is seen here working out her last years allocated to Slough Motive Power Depot and running these services to Marlow in company with sister engines 1447 and 1453.
Prior to her move to Slough 1421 had spent a short time at Reading, but had also served as far west as Plymouth's Laira Depot, doubtless working the Launceston and Tavistock rosters, although most of her working life had been in Wales, working branch line services through the Welsh valleys from Depots at Llantrisant, Ebbw Vale and Goodwick.
After thirty years in service 1421 actually ended her days in December of 1963 whilst back in the Westcountry again , being allocated to Exeter and used on the Tiverton Junction to Hemyock branch where a large milk production facility was based. "