John      (at 11:29pm, Tue 27th Jun, 17)

I measured my 1973 circular girder slot sizes.
See here.. http://www.nzmeccano.com/image-114750

John      (at 11:23am, Tue 27th Jun, 17)

Hmm. That won't work for a slot will it. Doh!

John      (at 11:22am, Tue 27th Jun, 17)

And the note says, was 1/4" now 0.250" so no diff except the limits were added. Have you measured yours Richard? It's difficult to measure hole sizes with a vernier, but a neat trick is to use your drill carousel and put the bits in the hole increasing the size until you get the perfect fit.

Richard      (at 11:11am, Tue 27th Jun, 17)

The hole size changes just seem to show that they used to have one dimension but then a tolerance range has been added. The biggest dimension didn't change in 1945, as far as I can make out.

John      (at 10:49am, Tue 27th Jun, 17)

The Binns Rd blueprints for part 143 don't show the hole sizes, however there's a note saying see Job 179 which is part 118 hub disc. The slot sizes are 0.250" x 0.172" (upper limit) & 0.244" x 0.166" (lower limit). There's also a notation that the slot sizes were changed on 9/7/45