Veering off-topic and I can’t believe I’m answering someone about the ATP!

The original development work to upgrade the ATP by fitting the PW127(?) engine was carried out on G-MATP, long before the Jetstream 61 name was thought of.
ATP set 64 was the first “Jetstream 61” production or otherwise. I don’t think G-MATP had all the “Jetstream 61” mods whereas set 64 did (I can’t remember what the other mods were now).

A similar analogy would be 146-300 G-LUXE which flew in 1991 with the LF507engine subsequently fitted to the RJ but G-LUXE could never be considered the prototype RJ, as it didn’t have all the other mods fitted.