[Mr70]

HI Fred
I believe those tire codes were applied to help the assembly line worker identify the tires easily and not have to squint to read the small tire size on the sidewall.Similar to Color Paint dabs on a part.They were applied in Bold large letters on the tread itself.
Just to add what Rick N. said:
*L8DS-was RPO-PL4 White letter F70X14
*L8DM-was RPO-PY4 White Stripe F70X14
*L9DM-was RPO-PX8 G70X14 Whte stripe-Elco
Used at All Plants,all time lines.
GoodYear-GoodRich-Firestone-Uniroyals-Generals were the 5 Manufactures.
I believe Coker is or was having a Tire sale on Goodyear & Firestone BB F70X14,the only Brands available today.
I may have a Lafferty Trunk emblem for you if you need one.
Rick