This is really turning out, thanks for the effort. Still can't zero my 72 Fremont 146xxx down to a single month, but pretty close. Fremont was really pushing them out too, they may have approached 9K a month at the time.
Any chance 71/72 Chevy Fremont trucks were the most plentiful?

The results so far seem to imply so, perhaps age just caught up to 67-70.
Without specific dates you can't tell how many were produced per month, but here's where my 72 C20 would fall for Fremont, as a guide.......146xxx would be Feb. or Mar.
135708 Jan.
153077 Mar. Increased by 17369
161544 April Increased by 8467
164046 April Increased by 2502
168790 May Increased by 4744