Yes thats it.... You already explore the logic behind.
Now the last thing you should think about is usage of other Displays.
The MaxTube (Green one with removeable diggits) got 12 Pins on each side. This are 7x Seg 1x Decimal Point and 4x Ground ( One for each Single Diggit )
Important to know.... The Segment and decimal Point is "shared" on both sides.... So there exist finaly 16 different Pins allover the two blocks. The other 8 Pins are youst shared to connect the 4 Diggit Blocks more comfortable.
If you now need for example a 5 Diggit Display you can order it in a shop.... Only important require is "COMMON CATHODE" . ( Anodes wont work)
A 5 Dig Display for example got 13 Pins normaly..... In that case you will connect the 8 Data Pins ( Seg Pins and Decimal Point) from ONE side of the Max Tube..... And you connect the 4 Cathode Lines from this side and additional the 5th cathode from the other side.
Same logic with a 3 Diggit Display.... You not need to work with the 4 diggit original Display from the Tube. If you buy a 3 Diggit one this should have 11 Pins Normaly..... Simply connect it to ONE side but let the "Dig4 Cathode" line empty.
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Last Note:
The Problem here is that mostly all Displays got different Optics (Size, form, Color and so on) . If you need in MCP for example 3,4,and 5 Diggit Displays then all should be from same manifactor and product line to get a simmular optic.
Thats why professional cockpitbuilders work completly with 1 diggit Displays.... They bough simply 200 Single Diggits and build the Blocks themself.
Problem here..... every single Diggit got 9 needed Pins ..... So a 5 Digit Display include 45 Pins. With a solder logic by hand this is scared
Here a PCB is needed..... so all the lines are on the PCB and you finaly got a connector with 13 Lines ( in case of 5 diggits) that is wired to your Max7219
Check this for example ......
https://www.silicon-ark.co.uk/7-segment-5-digit-led-display-pcb