You cannot use Baker A castor in medicinal castor because it is more likely to contain small amounts or ricin poisen. Medicinal castor is Bakers AA which has been tested for traces of ricin poisen. The extra test raises the price considerably. It can be used in model engines. I have used it in a pinch for 1/2 A engines, no problem. There is no process to remove the gum, except possibly the filtering which is done on all grades. All castor will gum up in time. Castor which has been blown, is drying castor which will dry and harden. This is made for the painting industry and is used to add flexibility to oil based paints. Perhaps this is the cheap castor that some called gummed?

IMO Clarance Lee is a good machinest, not an engineer. He has repeated a lot of myths in his articles. Peter Chinn however only reported what he knew to be true. Though since the industry seems to refer to castor oil as degummed, I wouldn't be suprised if he repeated this.