This pen is from the personal collection of Marcwithac - a generous contributor on FPN and vintage pen collector with exceptional taste!
This No. 1 safety pen was made between 1920-28. The enamel overlay is rather special, and not easy to acquire today! According to Collectible Stars I, the size 0 enamel overlay pens score a 10 out of 12 in rareness and are considered by the authors as a "once in a blue moon" find! The enamel overlay was made in blue, yellow, green, and also a beautiful white colour with flowers on it.
The No. 1 pictured here has the correct size and design "Simplo" nib from the early 1920s. Notice that this pen doesn't have a clip. I don't think these models came with clips because I haven't seen any pictures of enamel overlays with them. The material under the overlay is hard rubber, just like the common safety fillers of the time.
The No. 1 was quite a small pen at about 132 mm posted.
Safety fillers are a wonderfully intelligent filling system if you ask me. The nib unit rests inside the barrel when the pen is capped. After uncapping, a turning knob at the bottom of the pen is used to extend the nib unit and lock its collar against the section so that no ink can flow out of the barrel. Because of the fact that the nib unit is always submerged in ink, this pen was advertised as never suffering from hard starts!