No adjuster access in the backing plate or drum face is not that uncommon in the industry. You need one of these shoe adjuster gauges where you flip it one way to gauge the drum then flip it around to adjust the shoe up to that dimension. This works great but if your drum has a wear lip on the edge you’ll never get them adjusted correctly. I use this tool even on vehicles with an adjuster screw access hole just to get the shoes adjusted up close to the drum then I do a few clicks of the start to run them a bit tighter. I find the tool usually gets you right on the money.
As mentioned above, most of the older 1/2 ton trucks without access in the backing plate have a knock out in the drum to punch out for adjusting however, if the truck has received a set of China's finest aftermarket brake drums in its life they probably do not have the knock out provision.
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