David Haugh's War Wheels webstie includes the T4/M1 Armored Car Index
From that site one can download his .pdf datasheets for both the T4 and M1 armored cars, both of which models Haugh claims the Cunningham Co. designed and built. He also claims that only 10 M1 armored cars were made, as well as the two earlier T4 cars. Their War Dept serial numbers ran from W-60114 to W-60125. Haugh strongly implies that the two T4 armored cars became M1 armored cars, thus a total of 12 M1 cars, with their consecutive serial nos. That's the way I read it anyway.
As for that photo that Haugh shows in his other M1 armored car site, I'm not certain the vehicles much beyond 8 or so are M1s. The more distant ones have long hoods and shiny headlights, too, but what are those long gun-looking things? Their not tanks at that time period, because the U.S. Army had not yet developed turreted tanks with large guns.
Great discussion site!