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In this drawing 4 of the holes have the top left quarter filled in black.

What is the meaning of that? It it some special type of hole?

The drawing is from a Waveguide and in the specs it says

Flange type FDP32 (Cover) 

So I'm thinking it might be some way of mounting the cover.

Is there a standarized meaning for the enter image description here symbol?

enter image description here

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    $\begingroup$ I am wondering if they are in different patterns/groups. I just noticed the hole positions are basic dimensions. Is there a geometric tolerance callout elsewhere in the drawing, especially one with a composite (feature/pattern) true-position callout? (like this): $\endgroup$
    – Pete W
    May 19 at 12:31
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They are just identifiers for different types of holes. If the same symbol was used for every hole then every hole would need the size specified beside it if there was more than one type.

With identifiers they just have to list the legend:

enter image description here

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    $\begingroup$ But all the holes are the same (6.35mm diam), which made me think that maybe there's something special with this 4 holes to be distinguished from the other 6. $\endgroup$ May 19 at 10:02
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    $\begingroup$ @SembeiNorimaki Waveguide flanges sometimes have a set of holes for joining with screws and nuts, and another set of holes to insert dowel pins for precise alignment. I'm not 100% sure if this is the case here or not, but it's my best guess $\endgroup$
    – llama
    May 19 at 17:21
  • $\begingroup$ @llama If that's the case the drawing should call out different tolerances for each. Holes for alignment pins need to be reamed to a precise diameter. Thru holes for bolts need more play. $\endgroup$
    – J...
    May 19 at 18:25
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    $\begingroup$ @J... yeah good point, I'm starting to think this is just an unintentional artifact of someone making 2 different hole patterns in the drawing software $\endgroup$
    – llama
    May 19 at 18:37
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    $\begingroup$ @J... Just posted an answer, it seems that Chinese manufacturers refer to CPRF flanges as FDP with a different numbering series for some presumably arcane reason $\endgroup$
    – llama
    May 19 at 18:58
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In this particular case it appears to have no meaning, I think someone just didn't notice or care when making the drawing. I'm guessing they made two hole patterns in the software they were using and it automatically labelled them differently.

After doing the digging to figure out that FDP32 seems to be a name used by Chinese manufacturers for CPR284F flanges (using this table to see that FDP32 is used for WR284 waveguide, and the column is labelled "FDP (CPRF)"), I found this drawing c/o ATM microwave:CPR284F flange drawing

Some of the dimensions don't match exactly when you convert to mm, interestingly, but more importantly the holes are all part of the same pattern. Note about waveguide sizing: WRxx or WGxx refers to the dimensions of the waveguide itself (so determines which frequencies it can carry), but there are many different types of flanges which a given waveguide size can be used with.

Here's another Chinese manufacturer's drawing calling it FDP32 but explicitly calling the 10 holes part of the same group (from here)

FDP32 flange drawing

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    $\begingroup$ Probably the FDP drawing calls 6.35 because they do it cheap and just use a plain 1/4" twist bit with a bit of hoping that a cheap bit and sloppy setup will get them that six thou. $\endgroup$
    – J...
    May 19 at 19:03
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I don't think there's a standardized meaning, if there is I can't find it in glossaries.

If I had to guess I would say that they could possible be an extruded hole? Something like this FDP 32 model

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