Well it seems I can not find the stuff I remember but instead find stuff forgotten.
Just ran across a piston/sleeve for an Enya .35 that also looks like the ones posted.
The bore is approx .801.
The pic is of the .35 an project I dug out to finish. Stroking it with a .45 6001 crank.
I do not have a .29 dissembled to check but it is a possible your sleeve is for an Enya .29.
Ah! just dug this out, a NIB Enya .29-IIIB Super Typhoon model# 5103 with specs sheet.
Bore .735( you measured your piston at .738)
So far no dead on match.
The closest measure between indicates the .30 OS but if memory serves right ENYA had several bore/stroke engines in various sizes.
There are no by pass holes in piston or sleeve on these 29s and 35s.
I think the OS .25 did have holes.will be awhile before I get around to restoration of this size engines.
Brian Hampton from Adelaide(downunder) has good knowledge of the Enya.
Tell him I sent you!