I have a Foster anvil that has seen very little use but has no date.
With respect to the "extra" initials, let me suggest an explanation, by analogy to axe makers in Ontario (set out at greater length in my book). On some axes, especially those made in larger shops, an extra set of initials appear. They are the initials of the journeyman axe-maker who forged the axe. In these larger shops, there would have been several forges and several journeyman. Axes were sold with warranties and it was necessary to be able to identify later who made an axe which failed.
I suggest it is the same for anvil makers. I can identify some of the journeymen whose initials appear on Ontario axes. Some research in English directories might suggest the identity of the men whose initials appear on Foster anvils. Could the W initial stand for Wilkinson? Gary E. French