The following information was uncovered by spankahontis, milytar, Jaxartes and ShoC.
Mk5 shields are available in Behemoth, Steady or Dashing configurations, just like the regular engines (Mks 1-4). They are offered for sale in pairs, in exchange for either of two collections of rare items found in unique lockboxes.
If you're playing the campaign game start, try to find the Relics in Maelstrom before the plot has you attacking Xenon stations to avoid accidental destruction of the lockbox's contents. See SonOfToastie's explanation here.
Whilst the items are stored within indestructible lockboxes, their locks can be opened by stray fire or explosions if we are in the same area (same zone or neighboring zones) as the lockbox. If we don't find and collect the item after the lockbox is opened and the item is released into space, there's a chance they themselves could be destroyed and lost. So be careful with explosions, and avoid shooting the expelled item after the final lock is opened!
The Skunk can equip two shields simultaneously and the completion of either mission allows the purchase of both. The duplicate mission can be considered a chance to reconsider an earlier choice or a second chance in case of misfortune. Although the MK5 shields are given to the player for free, old shields are still refunded at normal rates (as with any upgrade). This means that when upgrading from MK4 to MK5 shields you will still technically gain a few million credits, despite not having chosen money as the reward.
In freeplay starts there is no limit to how early one can get the shields, as all pieces can be obtained right from the start and most are in safe areas. As such it is possibly to go directly to MK5 shields from whatever shield you started with. The only difficulty one may encounter is with the pieces in Maelstrom as they are near Xenon bases.
In general, whether Long Range Scanning is required for the Exploration items or not, it can help since related lockboxes or loot crates have different ping sounds and colours (with a top-end scanner) than those of asteroids and ships
The "A Piece of History" mission calls for the gathering of 5 Ancient Relics and the crafting of the "Ancient Relic Collection".
The "Treasure Hunt" mission calls for the gathering of 5 Kha'ak Relics and the crafting of the "Kha'ak Relic Collection".
After returning to the NPC offering the respective missions, you can either exchange the item for the opportunity to purchase Mk5 shields or sell them for profitsss.