I originally believed that two of my Eagle pens were blow fillers based on no visible filling mechanism and a hole in the end of the barrel. However, I came across the ad seen below in Geyer’s Stationer. I never came across any Eagle blow filler ads. So, these pens seem to be the “self-filling” fountain pens described in the ad. My examples are metal with enamel painted to resemble hard rubber chasing. The feed is hard rubber. Simply remove the section with attached sac, “press the bag”, and fill, then replace. The hole in the barrel is there to prevent pneumatic compression of the sac when the section is pressed back into the barrel. [although this approach by Eagle is admittedly low quality, conceptually I suppose the filling system is like the Parker 51 Special, but with the sac guard removed.]
Shown below with remnants of the original sac. Pull out the section with attached sac. then just manually squeeze the sac to fill, and put the friction fit metal pen case and section back together.
Upper right of photo shows hole in end of barrel.