Actually how a filter is designed can make a difference on how it performs. As stated here someplace, pleated filters generally are deigned to filter out to in. Depth filters (kinda what the sock filter is) GENERALLY also work out to in. BUT CAN be run the opposite direction. It all comes down to the physics of the situation. Basically as you pressurize the outside of the filter it will compress to the core, and at some point it will plug. The core is generally designed with some sort of material (plastic/metal) keep it from collapsing into a ball of goo. Pressurizing IN to OUT will force the filter as it loads towards the housing container and eventually it will expand to fill all available space as it plugs up and "grows". This can make it nearly impossible to remove. How filters are made (in this case depth filters) it is generally way more easier to create a graduated filter by having looser material on the outside and as you work towards the core, denser material to catch finer particles. This allows the filter to work longer and more effectively. Conversely, if you pressurize the crore you have way less control over how the filter will react as it loads as it will EXPAND, OPENING up channels for debris to pass THRU the filter completely (filter bypass). This information is directly gained from years as a test engineer/filter designer for a filtration company making filters for pharmaceuticals, potable water, semiconductor industry and the like in the mid 90's. I won't speak for the logic used in the 50's when these parts were designed, but that is how modern filters are designed.