St.louis Postmark (Of 1885) Followed By A Huge Letter "W"

Hi all, what does the big letter "W" following the (1885) postmark mean? It is not too common I'd say as I did never notice such before. Thank you in advance

Often on RPO mail, the W or E postmark indicates the direction of travel of the train on which the letter was postmarked. This one being postmarked in the "W. St Louis Station" likely wouldn't be that, in particular because 15th street is East of the train station.

Cole does list a big W cancellation (Lw-22) from West St. Louis Sta. from 1873 but it is seriffed and yours is not. But the meaning of the cancel is probably the same, the "W" postmark comes from the "West" station.

Thank you again, ZebraMan. This is not the first time you enlightened me.
I appreciate your help very much.

My pleasure. Always happy to help share my opinion when I can.