A rough sketch to how I'd register subtypes.

subtype_registration_example.rb

class Thing < Struct.new(:color)
  @@things = {}

  def self.parse description
    type_name, color = description.split
    @@things[type_name].new color
  end

  def self.type_code code
    @@things[code] = self
  end
end

class Chair < Thing
  type_code 'C'
end

class Phone < Thing
  type_code 'P'
end

describe Chair do
  context 'a parsed chair' do
    subject { Thing.parse 'C brown' }

    it { is_expected.to be_a(Thing) }
    it { is_expected.to be_a(Chair) }
    it { expect(subject.color).to eq('brown') }
  end

  context 'a parsed phone' do
    subject { Thing.parse 'P yellow' }

    it { is_expected.to be_a(Thing) }
    it { is_expected.to be_a(Phone) }
    it { expect(subject.color).to eq('yellow') }
  end
end