In contrast to the slightly larger MX500 type shell, the PK type shell is popular as smaller type shell.
To understand PK types, you should first read
@WoodyLuvr's hard work on the
"History of EARBUDS (Key Models Timeline)" post.
SILVER AGE
As first PK type shaped earbuds, the "2006 - NTT DoCoMo New Variant (16Ω)" which was an accessory of the NTT DoCoMo feature phones was quietly released in Japan. This is now known as "docomo earbuds" and their driver is known as "docomo driver".
Next, the "2006 - Yuin PK Series (PK1 150Ω; PK2 16Ω; PK3 32Ω)" were released in China, where there is no concept of design rights, which was a copy of the docomo earbuds shape. The Yuin PK1 150Ω became especially popular and this series is still on sale.
The driver for the Yuin PK1 150Ω is called the red film driver, and it is also sold as the red film DIY PK1.
And a modified version of the Yuin PK1 is still available as "2008 - Yuin OK1 (150Ω)".
Next, a slightly larger version of the PK type "2013 - Sunrise Dragon 2 aka SWD2 (120 ohm)" was released from Sunrize and the anniversary edition is still available.
MODERN AGE
The following 2 earbuds with the PK type of "docomo earbuds" shape and with the drivers "docomo driver" were released from Shozy who found that "docomo driver" sounds great and are known as masterpieces and are still sold today, with occasional shortages.
"2016 - Shozy Cygnus (18Ω)"
"2016 - Shozy BK Stardust (16 ohm; NTT DoCoMo New Variant driver)"
And the successor to the Sunrise Dragon 2 aka SWD2 has been released and is still available as "2017 - Simphonio (Sunrise) Dragon 2 Plus aka SWD2+ (16 ohm)".
Although not in this timeline, the "2017 - NiceHCK DIY PK2" using the "docomo earbuds" or "docomo driver" as parts was released but sold out on Taobao and Aliexpress.
I think that other than this NiceHCK DIY PK2, no other products with the DIY PK2 name use the "docomo driver" after this.
Many kinds of PK type earbuds are released...
Finally, "2019 - NiceHCK B40", a slightly larger version of the PK type, is now on the market.