Through an arrangement with Mayking Records, MPO (Moulages et Plastiques de l'Ouest) pressed most of the vinyl for independent record labels in the UK in the 1990's, including Tupelo . At the time MPO only pressed black vinyl  (now they also press colored vinyl and picture discs ), and any Nirvana colored vinyl or picture discs with "MPO" written in the matrix codes were pressed by Orlake which received metal plates from MPO .
MPO did not cut records in-house. Most of the records they pressed for Tupelo were cut by Utopia in the UK . Utopia, and other mastering studios which cut records pressed by MPO, only wrote the catalog numbers and side information around the outer edge of the lacquers to keep track of them during the plating process. This part is trimmed off of the stampers and not transferred to the records. The actual matrix codes were written by MPO.  Only the mastering studio names or engineer signatures were written in the run-out grooves by the mastering studios. All the matrix codes on the Nirvana releases MPO plated and in most cases also pressed, including compilations, seem to have been written by the same person, as shown in image 1. They are consistent regardless of which mastering studio performed the cut. The codes were written onto the lacquers before plating, because they are deep and smooth.
1. Top: Heavend And Hell Volume One (ILLUSION 016) cut by The Exhange. Bottom: Sliver (TUP25) cut by Noel Summerville at Utopia. Both matrix codes appear to have been written by the same person, except for the mastering studio signatures to the right. Especially note the trademark superposed numbers after the side information, and how the various numbers look identical on both cuts. (The green Sliver singles were not pressed by MPO, but they made the metal plates and also pressed black test pressings.)
Mayking was a full-service production company. A record label like Tupelo could deliver a master tape to Mayking and they would get the lacquers cut, get test pressings made, make sleeve proofs, print the sleeves, press the records, package them, and deliver them to the final destination. Mayking provided finished product for nearly all the independent labels in the UK. When Mayking started to press CDs in-house in the early 1990's, all the various elements involved in vinyl production were subcontracted to outsourced suppliers. 
All of Mayking's black vinyl orders were to be be pressed by MPO, and in return MPO would not press any records for any other company in the UK. This agreement stopped MPO from going straight to Tupelo and offer to press for a cheaper price. As MPO only pressed black vinyl, colored vinyl and picture discs were pressed by Orlake in the UK. 
The finished product was delivered by Mayking. The sleeves, at least for Bleach (TUPLP6), were printed in the UK on Canadian cardboard using German ink. Even though the records were pressed in France, the retail records were, by EEC rules, manufactured in the UK as this is where they became finished product. 
Mayking ceased to exist in 1997, when they were bought by DOCdata. 
This partnership is the reason Mayking's logo can be found on most of the Tupelo test pressing labels shown on this website, even though Mayking never pressed a single Nirvana record. MPO was only mentioned in small letters. The only reason the MPO name even made it onto the labels was because MPO wanted greater recognition for the quality of their product. Their quality on vinyl pressing was only surpassed at the time by maybe one other company that specialized in classical music and charged twice the price for vinyl pressing. 
White label test pressings were done only upon request from the record label. If white label test pressings/ promos were ordered, the first three would generally always have the Mayking test pressing labels.