Do you have any contact with the seller, who may be able to tell you?
It might just have been for another locomotive he’d started building, sold unfinished and forgot to pack this casting with the rest.
The problem with enquiries like this, apart from risking rather sour responses, is the object is likely to be identifiable to species level only by somebody who has used an identical one.
The only other way I can suggest is trawling through the Reeves on-line catalogue to see if it appears there; but with 62 different 5″ gauge locos listed, good luck with that one! The shape suggests outside cylinders and valve-gear, which might narrow the search a bit.
Or perhaps send the photo to Reeves and Blackgates to see if they can identify it.