It’s more likely that he’s just writing in Shorthand (or the Equestrian variant of it) since he often has the letters dictated to him.
 
They used Shorthand all the time in the 60s and 70s when someone was writing a letter while someone else was saying it (it was way faster to write it and keep up with the dictation). It mostly looks like squiggles, but each squiggle means an oral sound, rather than a letter, and can be read by those who recognise it. It looks like Spike’s using that.