Thank you for the reply. I was doing some further research and found out that if there is only one monitor attached to a system the number comes out correctly. Also I should have been clearer on what I am doing. I was verifying serial numbers on remote systems that I do not have access to but know the system and monitors are on. I verified this on a system with only one hp monitor attached.
Just wanted to add that this code is a bit broken, excluding zeros (-notmatch 0) yield some weird results. As an example, on my system the models DELL U2713HM and LA2405 becomes DELL U713HM and LA405 (the code works if I don´t exclude the zeros from .UserFriendlyName).