It is a spare boiler for a Dinorwic Port Class Loco. It came off No 1 when I owned it. I had the opportunity to buy the only spare boiler for a Port Class when I owned No 1 so bought that. I then had that one certified by the boiler inspector and placed that on No 1's chassis that way, the loco would have been out of service for a lesser time.
In fact, the boiler I bought, turned out to be No 1's original boiler. I think the one on ebay we worked out was off Dolbadarn.