Having seen them 'in the flesh' they are almost certainly for a 2½" loco. I did suggest 'Dyak' as a first guess, but I think they are probably for 'Annie Boddie' which is a 4-4-0 tender loco from 1933. The dimensions of the former plates are not exact but close enough.
PS I just happen to have a chassis for an Annie which requires a boiler ;D
Bryan, Hello again. Per Baggo's guess, these do indeed look very much like DYAK plates however the Dyak outside firebox has a somewhat shorter lower firebox than your plates appear to have. No matter, the builder could have made these long with the intention of trimming off later. A couple of other proportions seem to vary a bit from Dyak, the shape of the inner firebox crown for instance, but this could still be for a Dyak boiler. This is a Cad profile of the original Dyak firebox. The scale bar divisions are 1"
Thanks for the drawing. This boiler is related to Dyak as the pattern of the tubes and proportions of the plates are exactly the same but the boiler barrel is bigger on Dyak (I make it 3 1/4"). On my mystery plates the boiler barrel diameter looks more like 2 3/4".
Baggo thought it was for Ana Bodie (any Body?) which he has castings for so I was going to donate the plates and formers to him for finishing.
Bryan, I built a boiler for the Dyak a few years ago and its proportions made it a puggish looking little thing, short and fat. A 2.75" barrel would have improved it's proportions however one certainly couldn't complain about a lack of water & steam space. I never got any steaming feedback on that one though, IIRC the fellow I made it for sold it off to someone else and it's disappeared into the mists.