I already have my Middle Valley Maple curved Tees and short spouts ready for my 5/16 lines.
I'm waiting for the new curved Tees for my gravity lines.
I heard from Bob Carpenter at Middle Valley Maple yesterday and thought I'd pass along what he said.

Hi Dave,
Thanks for your interest. Our complete line of 3/16" fittings will be available around the end of September (that includes the 3/16" ARC tee with a 5/16" dropline).
We currently have the 3/16" Tee (all connections) and the 3/16" End-of-Line Hook. We'll also have the 3/16" Hooked Straight Connectors (and the 3/16" to 5/16" Hooked Connector in about a week. The one your looking for will be available later in September (tooling is currently being constructed).
We will also have a complete line of Antimicrobial fittings and pre-cut droplines for the 2025 season.

He also attached some info that shows the line up and said they'll have our web site updated soon.
Their code number for the 3/16 x 3/16 x 5/16 ARC-Tee is 170-043 for $30 per 100 ct.