In a perfect world with perfect economic, intorduction of the new and improved item would have probably resulted in phasing out of the old item and replacing it with a new one. But our world is far from perfect and here is an example - KM-34/KM-34D Hybrid mask. Introduction of the KM-34D mask sometime in late 1970s made original KM-34 obsolete and factories started to make new masks. At the same time they found that they were stocked with old KM-34 parts for which they already paid, but now did not need. Since both, KM-34 and KM-34D masks used the same outer fiberglass shell, it was probably decided to cut down on the production costs and make first batch of new KM-34D masks fitted with old KM-34 parts. The example that I have, pictures of which you can see below, has a KM-34 fiberglass shell fitted with a KM-34D rubber mask.

Information provided by ID tags of this mask provides us with a very interesting picture. KM-34 fiberglass shell bears production date of 1980, while KM-34D rubber mask was made in 1981. This tells us that new KM-34D design was ordered in to the mass production sometime between February 1980 and December 1981. We can further narrow down the timeframe by looking at early KM-34D masks. One of the KM-34D masks I have has KM-34D fiberglass shell marked as being produced in March 1981. Thus KM-34D was ordered for production sometime not earlier then February 1980 and no later then March 1981 - 13 month difference. I hope I will be able to see more early KM-34D and KM-34/KM-34D hybrid masks and narrow this timeframe even further.

 

Front view of the KM-34 mask. It looks like a bigger brother of the old KM-32 and this is not far from the truth.
Fiberglass shell has two ID tags. Information of the ID tag tells us that it's a KM-34 fiberglass shell of size 3. It also says its serial number - 4520.
Second ID tags says "OTK II-80", which most likely means the production date of the shell - February 1980.
Mask's ID tag located on the microphone bump (just like on the old KM-32 mask) of the KM-34D rubber mask - top line says that this is a KM-34D mask, second line gives us its size (3) and third line stands for production date of the rubber mask - sometime in 1981.