iOS: Exclude digits from the effects of small caps

Question

I'm applying small caps to lower case letters, but it appears that it also includes all digits. This is also true when I apply it only to uppercase letters.

Is there any way to exclude digits from the effect of small caps (like the label for wind speed/direction in the stock Weather app), or do I need to resort to an attributed string for that effect?

Currently I'm just applying a font to all label content:

extension UIFont {
    func smallCaps() -> UIFont {
        let settings: [[UIFontDescriptor.FeatureKey : Int]] = [
            [
                .featureIdentifier : kLowerCaseType,
                .typeIdentifier : kLowerCaseSmallCapsSelector
            ]
        ]
        let attributes: [UIFontDescriptor.AttributeName : Any] = [.featureSettings: settings]
        
        return UIFont(descriptor: self.fontDescriptor.addingAttributes(attributes), size: self.pointSize)
    }
}

I tried using kNumberCaseType and kUpperCaseNumbersSelector in the same vein as the above code, but I guessed wrong. According to the documentation it turned out not to be what I was looking for.