These are transistors with built-in resistors, also called 'prebiased transistors'. The Codebook uses 'dtr' or RET as it's shorter... Some have one resistor between base and emitter, others one in series with the base lead. Many others have both. To keep things simple, the series resistor is called R1 and the base emitter resistor is called R2. If both are present, then two values are given, R1 first. So 4k7 + 10k means that R1 (the base resistor) is 4k7 and R2 (the resistor between base and emitter) is 10k.