A debate has been taking place on another forum as to how 03121 came to be fitted with a bell high up on the cab front. Circumstantial evidence suggests it might have been acquired while based at Swansea for use in the docks but was retained when it came to Bristol. While trying to source photos, I suddenly remembered seeing your layout at Larkrail last year including Bath Road's other loco 03382. Great layout by the way!
Looking at photos of your models, I see 121 does not have the bell which was located just under the single air horn. Something else I noticed is that your model of 382 appears to have twin horns whereas photos of the prototype only show one. Granted your model is just a representation of the location where these locos worked but I'm intrigued as to why both models are different with this detail.
Roy