I have one boring bar that uses those, and already have a near-lifetime supply of inserts. (For a home shop, that's about 20, btw.)
There are a number of grades that are good for 316SS in the TCMT21.51, though I'd recommend going to a TCGT insert for it's sharper edge. OTOH, the TCMT and an appropriate chipbreaker might hold up better if it's cast or forged 316 and an interrupted cut.
Look for Walter WSM10 or WSM20, Iscar IC907 or IC908, Kennametal KC5010 or KC5025, and so on. There's a lot to choose from, but NONE of those will come as cheap as those Mitsubishi. Try ordering on a guaranteed test to see if they give longer life/higher productivity.
BTW, Sandvik Coromant will have the TCMT21.5x inserts in 1025 or (uncoated) 235 grades that work well, but they only have the them in the "UF" chipbreaker.