I answered my own question here. I needed to retrieve a list of all products, making sure to include inventory_item_id as a field in the result set, as indicated here: Product (shopify.dev)

How can I get a list of all product_variant_sku values and their corresponding inventory_item_id values? I need to update Inventory Levels in Shopify, but without some sort of cross-reference between these numbers, I cannot upload the available quant...