Piston identification inquiry

["Dan Curtis" <d.curtis@xxxxxxx>]

Upon recently looking at a rebuilt but still unused '69 390 that I purchased
a couple of years ago, I discovered the following number stamped into the
top of the pistons: 2381P.  The pistons are a .030 over and have a center
dish style casting with very slight valve reliefs on either side of the
dish.  Also, unlike stock '68-'70 style pistons, these have about a 3/8 inch
wide full piston height flat top ridge around the complete outer edge of the
piston whereas stock pistons have a very narrow full height ridge on the
outside of the piston with a deep valve relief and a substantial center full
height ridge. There is no high center section in these piston so they are
not at all like standard cast '68/'69 or '70 pistons.  The center sections
of these pistons are entirely dished, using the significantly wider outer
flat top ridge to reduce the relief volume.

According to the machine shop I bought this complete balanced and
blueprinted rebuilt 390 from, these pistons are supposed to be a 1970 center
"dish" style casting and they claimed they milled the block to be decked to
have the same deck height of .010 above these pistons, just like a '70 block
would be with '70 pistons.  The compression ratio was also said to be around
9.8 or so. I looked at the deck height at TDC and it is indeed just slightly
above the top of the piston to what appears to be a .010 deck clearance
above the top of the piston to a '70 spec deck clearance.

So my question is does anyone in AMC land recognize this number that is in
the top of the piston (2381P) of this dished piston design and if so, can
you give me a hint as to what this piston is?  There is no name indicated on
the piston other than the 2381P and .030 stamped into the piston surface.

I have fiddled quite a bit with a compression spreadsheet that was
generously sent to me by a list member. The only combination I could find
that produced a 9.8 compression ratio is if this was a 28cc relief 401 later
model piston in a '68/'69 block with '68/'69 rods and a decked block height
to .010 over the piston.  This is of course when using fresh stock 50.6 cc
'68/'69 heads which this engine has.

So does the description or the number stamped into the top of the piston
sound familiar to anyone? If so, can anyone shed some insight on this piston
mystery.  If so and you have any feedback, it will be greatly appreciated.

Best Regards,
Dan Curtis
Cell 602 317 2018
d.curtis@xxxxxxx