Actually more complicated than that. Within the Lilly branding, Regular (R), NPH (N) and L (Lente) were all available as either "Iletin I" or "Iletin II". Iletin was the Lilly brand.
"Iletin I" was beef+pork or any arbitrary mix (between 100/0 and 50/50 and 0/100) of beef+pork.
"Iletin II" was pork only.
Some folks who were in some way allergic to the beef kind used Iletin II. The pork-derived insulin was less likely in some to cause insulin allergies.
I think Novo/Nordisk had a similar distinction scheme. I think some brand (not sure what, one of the UK brands?) was available that was 100% beef for those allergic to pork kind.
Tim.