Need help identifying English teapot maker, please!

I acquired this china teapot about 15 years ago and have never found the maker. I have around 200 teapots, but no other matches this shape. It measures 9.25 across, from end of spout to end of handle, and 5.25 inches tall. "Made in England" is impressed on the bottom, and the pattern number is 2504. I cannot make out the two characters below the model number. I'm not expecting it to be worth more than any other mid-20th century English teapot, but I'd love to find out the maker.