I love me some Pacific Mail Steamship Co. cancels. They're loads of fun and always in demand ($50 at the low end, up into several hundred for the scarcer ships or more dramatic multi-strike pieces).
I gave a bit of a primer here in this post:http://goscf.com/t/43579
In that post I showed a very rare multi-ship piece, with three strikes of ALASKA and four strikes of ARIZONA.
In that post I surmised that these were canceled at a central office, and because the two words look similar (both begin and end with the letter A, and are only one letter different in length), a clerk grabbed the wrong handstamp, realized their error, and then re-canceled with the correct handstamp.
Plausible. Uncertain, but plausible.
Well, I just acquired my second multiship example. I know full well that I overpaid for the piece, but I just had to have it. I've seen a total of three multiship examples in my revenue collecting lifetime, and I put these in the category of "if you see one, you'd better step up and buy it, because who knows when you will see another".
It has 2 strikes of the ship MONTANA and one of GREAT REPUBLIC.
The ship MONTANA is one of the more scarce Pacific Mail ships, even just as a single strike; I have only found one example in all my searching.