Peter Wright Anvils are some of the finest quality anvils. The 1-1-1 on the anvil is the weight of the anvil in an old system of units. The first "1" = the weight in "hundredweights". 1 hundredweight = 120 lbs (if I remember right). The next "1" is the weight in tenths of a hundredweight, or 11 lbs, and the last "1" = the todd pounds, or 1 lb. So, the weight of your anvil then = 110 lbs + 11 lb + 1 lb, or 122 lbs. This was a popular weight of anvil for horsehoers and light-to-medium smithing.
Peter Wright anvils were made in England and imported into the USA in large numbers. They were popular with blacksmiths of all types as well as farriers (horsehoers). They are, IMO, a sought-after anvil by today's blacksmiths. In reality, there are few remaining anvilmakers and the prices charged for a new anvil are astronomical. For this reason, a good quality old anvil in reasonable shape is sought after.
The square hole you name on the anvil is the "hardy hole". Anvil tools with square tapered shanks fit into that hole. The "triangular piece" you mention is likely a hardy or cutoff tool that fits in that hole. Other tools for forming such as "swages" (for forming round work) or "fullers" (for necking and grooving or reducing the size of stock), or flatters, bending forks and anything else the smith needed to shape or cut the work were made up to fit into that hardy hole. The round hole is the "pritchel" hole- used for punching nail holes thru on horsehoes for themost part.
I have two Peter Wright Anvils in my shop. Both about the size of yours. The older of the two I got in 1964, from a quarry blacksmith's widow, and it was old then. I was a kid of 14, so the widow treated me right. She sold me a mess of tools with the anvil for 25 bucks. The other Peter Wright came from a friend who got it from his dad. His dad had been a working smith who died in 1952 or so, and that Peter Wright anvil came from his dad's shop. Both are fine anvils with a reasonable ring to them. IMO, one of theother attractions to a Peter wright anvil are the shape and proportions. They simply are a good looking anvil, known as a "London Pattern" anvil.
If you have one whose working face isn't worn like a swaybacked horse and whose edges aren't chipped too badly, you have a really good anvil. As for how sought after it might be, just go to any sort of blacksmith;s event (look up ABANA or "Art Blacksmiths" or check the "Anvilfire" website). At those events, you will see beat up anvils with swaybacked and chipped faces being sold off tailgates for some substantial prices. Used to be a dollar a pound for older anvils was the going rate, but I hear tell it is up to closer to three bucks a pound for an anvil in usable condition.
As for my own Peter wrights- I will not part with them. My son gets them. Take cvare of your anvil and do not abuse it. It is a fine tool in it's own right.