Flatpacks were intended to be spot welded to the PCB, and then conformal coated to hold them down physically. They were formed and soldered as well, and with this and a conformal coat ( a wide range of them, both solder through or remove totally before soldering) they were very popular as a way to get the size of a board down to fit a box envelope.
Beware that the pinouts of the flatpacks are NOT the same as the DIP versions, the power pins are in the middle, and not the common pin 8/16 or 7/14 as on the rest of the TTL family.
There were a few missile designs where the flatpack devices were bonded to a flexible PCB and then selectively conformal coated to protect the pads ( using a flexible coat) and then the whole lot was rolled up in a jig to fit the space envelope, and placed in a mould and then further potted in a solid resin, making a final structural part that had electronics, capacitors and such all in a cylinder that contained other missile structure. Low failure rate there, mostly from actual physical damage breaking the structure.
Still have the special Weller Flatpack tip I bought, and it is in a Solomon handle now. Made changing the failed flatpacks a 5 minute job, instead of the 30 minutes otherwise using a stainless steel wire and stripping each lead off the pad individually. Yes the plastic holder, you get 3 types, the ones illustrated, another with a 2 part clip together package and the third with the leads rolled around a bar to lock the pack in place. They are all a static dissipative plastic, though the insulation was measurable in Mohms, as you had a test jig you could simply place the whole package in as is, and it broke the leads out to a larger DIP pattern for insertion into a breadboard or an ATE system, so you could do a parameter check on the IC you were installing. Very fiddly trying to get 16 grabbers onto the leads otherwise without twisting them, and a twisted lead was a pain, as you could easily pop off the lid in trying to bend it straight, especially those in the top section. They did come in both ceramic hermetic with lids, ceramic with a glass frit seal, ceramic with side brazes ( only the larger number of pin packages though for MSI, which were almost a PLCC part with leads) and plastic moulded packages.
Strangely enough you get a modern adaptor that takes the flatpack, or a modern SMD part, and converts it to the DIP form, but with a sealed package. This is a compatibility part to replace old obsolete DIP parts with more modern SMD or old production flatpack parts, where you do not want to respin the board ( or certify it really ) to keep old things in military and aerospace running. There are even 24/28/40 pin adaptors with the inside being a FPGA and memory or an ASIC to replace obsolete DIP parts. Where else will you get a pin compatible new part to replace an obsolete 2708 OTP Eprom in equipment, and still have the same part interchangeable in the equipment. They were often used not as a memory but as an 8 input 8 output combinational logic block to replace a board full of TTL.
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