I know I saw an IWB with a retention feature somewhere but I don't recall where.

If I remember correctly the thumb depressed something during the draw.

Anyone have an idea of who makes them?

I'm forced to assume you are Block boy, since you didn't bother naming the gun make and model you need to fit the holster for.

AlienGear has that feature on their ShapeShift models.

Thanks Tortillia.

Yeah I pretty much stick with reliability over looks. But, yes, in this case a 43X.

Just for fun, I'm thinking of trying an experiment.  

Last week, Amazon sent me a left hand M&P Safariland 6378 instead of a right hand Glock 19.  A PSA Dagger fits perfectly and locks just fine.  I've got several ALS holsters that are right-handed, I could easily swap the actual ALS mechanism from a right-hand holster to this left hand holster. Then I would just have to fabricate some sort of IWB Mount that would attach to the Safariland three hole mount.

That would give me an inside to waistband ALS holster.

It would be just for fun. If I do it, I will post results.

Also Safariland 517 Pro-Fit IWB holster.  It uses GLS retention.

I think this was the one I was thinking of.....

https://safariland.com/products/model-575-iwb-gls-pro-fit-holster-model_575