internal locking mechanism

Thanks for writing in! The handing for most Andersen product is usually determined from the outside looking into the home.

If that doesn't help, I would suggest pulling up photos of the images side by side and taking a look at the details of each of the parts in your photo in comparison to your existing tandem lock. For example, in our photos the installation holes on the 38-149 are facing downward, the installation holes on the 38-150 are facing upwards.

Please let me know if you have any questions.