Thank you too.
On the first batch of 11 slot (511) all screws were 4-40.
On the second batch the module mounting screws, rear panel mounting screws and rack mounting bracket screws were all 4-40. Top and bottom panels to side panels mounting were M3 (countersink).
On the third batch that I have now the module mounting holes on the front are 4-40 but the rest is all M3.
The rest of the racks such as 501, 503 and 506 all have UNC 4-40 throughout.
Kind regards,
C