It sounds like your anvil is an early farriers anvil as well. I agree with you, 300 pounds seems awfully big for a farriers anvil. Even mine, at 181lb, seems big for a farriers anvil but I'm now convinced that is exactly what I have. I'll be honest, I had never seen the early English farrier anvils in AIA until you said what you did about page 80. The only farrier pattern I was familiar with was the modern cliphorn. I don't know how long the English made their farrier anvils but I do know the "modern" farrier pattern (cliphorn) anvil came on the scene around 1882.
Unfortunately, you may never know who made your anvil. I have a couple of anvils that I would LOVE to ID but there just isn't enough info for me to do it. Some people would say "who cares... it's a tool, use it!" but as a lover of history I like to know as much as I can.