Far from an expert, I have had some plastic grips from the past. There are more or less two types of plastic grips. One, the Franzite type, are "hollow" or shell type grips with reinforcing ribs on the inside of the grips. These are prone to warp and pull away from the gun frame, also might shrink slightly with age and use.
The other is a solid type of grip, sometimes with a rosewood backing (Jay Scott). These are the better of the two, less apt to warp.
Current grips are of better material, such as the fake ivory and stag from Ajax or Eagle.
There is one old type of grip that is very yellow and solid that is made of a material called Catalin that is a very good synthetic material.
Can you find any markings on the grips?
P.S. Pictures help. Both sides.