I agree fully with cfrphoto. He is spot-on as usual.
There is nothing altered about your stamp. It is a perfectly legitimate Type III coil. The watermark will tell you which exact stamp you have. It went through one of the mechanical affixing machines of its day. One of the most common affixers was made by the Multipost Company of Rochester, NY.
Here is a picture of two similar affixers, the left is labeled "Made by Multipost Co. Rochester, N.Y." and the right one is labeled "Commercial Controls Corporation of Rochester, N.Y." but really the same item. The small button on the front is the screw-on cap for the water reservoir. Pushing the plunger down moistens, cuts and affixes a coil stamp.
Patent listing on the back side of the Multipost affixer:
Business reply mail card c1930s:
2 covers mailed by Multipost with their affixed stamps:
Close-up of the 2 stamps from the covers above. Note the perforations are knife-cut and at the same slight angle at both left and right. Depending on the adjustment, stamps with 3 straight edges and one fully perforated side can be found.
Your original stamp is cut more straight than the Multipost examples, but still shows a slight slant evident in the red at the end of the perf tips at the lower right. It may have been affixed by a different brand of affixer, who knows, but the concept is still the same. Absent a cancel, yours was likely soaked from an unused prepaid reply envelope in hopes of later personal use.
Multipost also made more complicated mailing systems including privately applied cancels, too distant a tangent for this thread.
The Multipost technologies were largely replaced by meters. Some collectors consider the shortened knife-cut perforations to be damage while others consider it evidence of the evolution of mailing technology.