The later “vintage” Hohner melodicas used the plastic plug as shown in your photo. It is a simpler, lower cost solution compared to the internal, sprung-metal flap with button and foam seal that had to be riveted to the back inside cover on the older models.
You can buy the plastic plugs on one of the Hohner websites, but I can’t really see the plugs failing too often as they seem to be made of reasonably good quality materials and are of a reasonably good quality design.
However, I lament the cheapening of the design, even if the old metal flaps are harder to service and bend to form a really tight seal.
The main problem I have with the plastic plug is it is harder to use. I like the simplicity of just pressing a button to release the water.
All this means to you is you have a later model instrument. The only question is did Hohner cheapen anything else inside as part of the engineering change order. I don’t think so. I have many Hohners of the same model with both types of water valves, and they play and sound pretty much the same.
Welcome to the forum.