The later type were black to avoid exactly this. You were instantly drawn towards them in less than no time whatsoever and actually installation was better than instant! Easy to order too as if you ordered them next week they would arrive yesterday!
I've got a feeling....may be wrong...but I think you may be referring to the old Leyland holes. A completely different thing and you will find genuine Westfalia screws are not compatible....probably even the wrong thread. I occasionally use to see the old Leyland holes when travelling on Sheffield buses when I was a lad.
That's it...I knew someone would know. I think Leyland and a lot of other vehicle manufacturers used to fit them through back in the day. I understand that at one time Whitworth holes were considered the finest you could get.
Seem to recall that I was able to purchase something like the screws you mention at a ships chandler's albeit it in stainless.
Indeed they did and there was subsequent formula derived for calculating the volume of large London venues based on the Blackburn holes which I believe were quite difficult to quantify due to their size. The final tally was circa 4k I’m told. But as stated elsewhere this was pre metrification and it is unlikely they were German imports.
I do have a small stock of 3mm Westfalia holes, if anyone's interested. Used to attached the musical crumpet toaster. Loose packed, £14.95 each, posted.
Any good? https://www.ebay.co.uk/itm/291240735809?hash=item43cf4cf441:g:c24AAOSwfcVUEXlj&var=590393493762
I've used a selection of various sizes of these in our '73 in numerous places: https://www.allfix.co.uk/productgrp/self-tapper-raised-countersunk-recessed-a2-stainless-type-ab
