I think I may have finally found your pen, with caveats. Go to
this page and scroll down to the header that says "SHEAFFER'S 'LIFETIME' CARTRIDGE PENS". You should be able to match your pen, look at description and then click on the link in the 3rd coloumn that says "More" to pop up a photo. If the barrel has 1250 stamped on it, it may be a cap from one version mismatched on the barrel, but still from that general line of pens. Hopefully with all of this information you can get the pen in action as you see fit. You should also
look here to see some of the history of this general design of the Sheaffer inlaid nib lines.
Cheers,
Jon