Based on Rich's note above (thanks Rich!) I finally dug through "The Grades" paying attention to the fine print. Here's what I came up with:
V grade - 201 with vent ribs
G grade - ??? Assume same as V, so 201
D grade - 3% of 9346 = 280
C grade - X% of 697 + 447 = (assuming 3%) maybe 34
B grade - 20
A grade - at least one (pictured in TPS)
AA grade - 1
A1 Special - 2
If my math is correct I come up with 740 guns with vent ribs. There are almost certainly more than this but it looks like (based on all that we can know currently) around 1000 or fewer Parkers were built with vent ribs.
What gives me pause is a case in point. I own a 1927 DHE (link to photo album below) that is not in the Serialization book and for which records (at least on our search feature) do not exist. It seems reasonable as the TPS authors did to project the same percentage of hammerless barrel guns without records to have vent ribs too. Hence, applying the 3% to all D-grade guns and guessing that G-grades have the same number of vent rib guns as V-grades all seems reasonable. But the truth is we'll never know.
I'm not a trap shooter but I'd like to believe the original purchaser of my DHE with vent rib back in 1927 was one. The butt stock dimensions are not as upright as purpose-built "Trap" guns ( 1 3/8" and 2 3/8") and it does not have a beavertail fore end. With 13 3/8" of wood to the pad I'd like to assume the gun was ordered to have a pad and was built that way. Only more recently (about 25 years ago?) the last owner had a new pad and spacer put on and had the pad faced with leather - a nice touch. I'm going to visit the nearest trap club tomorrow and put a box or two of shells through it just because. Anyway, here's the link:
http://parkerguns.org/forums/album.php?albumid=805