Last sibling from a series including the WL500, WL550 and T-550, all released between 1970 and 1973, and complement to the L-308 or L-309.
The T-310 however was export-only and had a feature unavailable on the others : Dolby FM !
This is Dolby B, allowing not only to receive (upcoming) (potential) (maybe) Dolby FM broadcasts but also to decode tapes recorded with a tape loop added to the terminals which also include a 4CH output.
The T-300 is identical but for the Dolby FM circuit and dolby/tape switches ; both retain the 4CH and two multipath outputs.
There also are a T-310U (US version of the 310) and T-300V and probably a few more versions - Luxman is Luxman is Luxman.
Circuit-wise, the T-300 and T-310 are 4-gang FM tuners with dual-gate MOS-FETs front ends, five-pole Phase Linear and Linear Delay filters placed between three ICs and the IF amp, hermetically sealed coils in the MPX section for temperature stabilization, plus FETs controlling the audio gates.
The 3-gang AM section has three IF stages and 18dB /octave switchable filters and voltage regulators.
A chronology of the entire series with nudies and measurements here.
A real T-300 here and a T-300V here.