As mentioned IAS hold without autothrottle is only a climb or descent mode.
The cruise mode is ALT with speed manually controlled by power.
Early versions of the F27 had a very primitive auto pilot. Then along came the Smiths SEP2. Jokingly referred to as "seldom ever performs twice".
In fact, with proper maintenance it was quite capable of holding airspeed in climb or descent, but tended to oscillate a bit trying to be so accurate.
Most pilots drove it up or down on the pitch switch, which had two positions for low and high response.
The Yanks considered that autopilots in turboprops were for pussies. Real men hand flew, so most F27s that went Stateside had them removed.