Rab,
I've been on a bit of Akira Miyazawa kick lately, but the records that I've been focusing on were made just after the period that you're looking for. That said, I think Miyazawa -- much like Akiyoshi & Watanabe -- is regarded as a pioneer in Japanese jazz.
To give you a sense of his age relative to other well-known Japanese jazzmen, Miyazawa is at least 12 years older than his sidemen/collaborators on Bull Trout (Victor, 1969):
- Miyazawa - b. 1927
- Masahiko Satoh - b. 1941
- Yasuo Arakawa - b. 1939
- Masahiko Togashi - b. 1940
I haven't heard any of Miyazawa's work that precedes Bull Trout, which is probably (?) his most highly-regarded album. Even so, I thought I'd toss his name in the ring, because there might be some interesting stuff that does fall into the mid-Forties to late-Sixties timeframe you describe above.
Here's Miyazawa's Discogs entry (all credits, both leader & sideman).
The first thing that catches my eye: Lots of cheesecake covers.
My assumption: It seems like there was a flowering of creative and distinctively different jazz in Japan that began in the very late-60s. (This actually paralleled things that were happening in Europe.) Jazz without an American accent. In general, before that, it seems less unique, largely derivative.
But, like I said, this is just an assumption. Really, I haven't done enough listening to pre-1970s J-Jazz to know whether my assumption is wrongheaded.