Hi Heidi, welcome to the forum!
There are several places you can look on the piano for the serial number. On the plate (harp) directly under the music desk, it could also be stamped into the case up underneath, some manufacturers stamped them directly on the soundboard. If you still can't find any numbers, it may be stamped inside, which would entail removing the action.
I can give you a little information about the PC Weaver company. According to Pierce's Piano Atlas, the company was established in 1901 by E.P. Johnson. There were factories located at Elgin and Ottawa, Illinois. Johnson also made the Curtice and Seybold pianos. The Ottawa factory was closed after 1926. Pianos manufactured under the PC Weaver name show serial numbers starting in 1901 with #15000, and the last year of manufacture as being 1921, with serial number starting that year of #32000.
I hope this helps you in some way. If you find the serial number, post it, and I'll be happy to look it up to see what year your piano was manufactured.