I checked the number on the motor currently in my '64 Deluxe with that on 
the warranty card (found in the Service Booklet).  The book shows engine 
# 0301353 and the current motor is # 0590821.  I assume that this is the 
"G" series 1500 (there is no "G" stamped on the case). 
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in 64 the motor, whether the usual 40hp 1200, or the optional 1500, did not
come with a letter before the number.   the 1500s had a six digit number,
where the 1200s had a seven digit number.  In aug 65, with the 1500 as
standard, the case was stamped with an H.   does your bus have a "1500"
chrome emblem on the back engine lid?  or on the space between the engine lid
and rear hatch?

0 301 353 -  ~ early sept 63

0 590 821 -  ~ early june 64

what is your chassis number?

I'm curious as to the date of manufacture of this motor (and question why 
it is in such a low mileage Bus).  It could be that the PO used the 
previous motor to drive an external generator (or other contraption) and 
put a few too many "hours" on it.  There are about 16 holes in the floor 
above the motor and a manual throttle cable (handle mounted to the parcel 
shelf) along with the double pully and slot for the belt up through the 
floor.  It also has what I think is a rev-limiter or governor mounted 
under the 28-PICT carb.  It has the markings APG, Made in W. Germany, 28 
DR 1,5 on the face plate. 
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im more surprised to find an original motor than otherwise, no matter how low
the milage.    I was amazed to find that my 69 bug came with the original
(needing rebuild) motor.   people could (can) kill them in a few years, no
problem.    especially with the funky generator set up you described.  oh,
and yes the bus would have come with a governer to prevent overrevving.

hope this helps