I was given a stack of interesting material from a coworker today, and I found this entry from the AT&SF Railway’s SFRD Circular No. 20-C, dated March 30, 1953 for Code of Rules for Handling Perishable Freight, which sheds a little more light on the approved method for icing reefers:

3. Icing: (A) Cars should be iced to capacity, using 30% salt with both initial icing and reicings, coarse ice in end bunker cars and crushed ice in overhead bunker cars.
(B) (1) Initial icing of end bunker cars should be performed as follows:
Fill bunkers approximately one-half full, then chop ice down to make a bed for salt, then add 30% of total amount of salt; add 12 inches more ice, make a bed for salt and add 25% of total amount of salt; add 12 inches more ice, chop down to make a bed for salt and add 25% of total amount of salt. Then fill bunkers and add balance, or 20% of total amount of salt, tamping this 20% salt thoroughly into the ice.
Rule No. 145-C (Concluded):
(2) In reicing, old ice should be broken down and one-half of the required amount of salt placed on old ice, bunkers should then be filled, then one-half of the salt placed on top. This salt placed on top should be thoroughly tamped into the ice. In reicing, hatches in one end of car only should be open at a time.

Further along in the pamphlet under the section entitled Protective Service Against Cold, which specified methods for handling perishables that must not be allowed to freeze, I found the following newly added (for 1953) instruction:

Note: Reicing with chunk ice is to be placed in body of car on top of barrels or in bunkers of car as indicated by initial icing shown above. Waybill notation covering reicing in body on mixed shipments of barrel and package beer must include: "Place on top of barrels only."
Applicable on shipments of beer and ale:
On shipments transported in a car not equipped with ice bunkers and without a heater, shipper must must specify on bill of lading and shipping instructions and agent must place corresponding notation of waybill: "No heater in car."

Now you know!