Notice that eBay one is a mid 50s Hartrick Large Stamp Type 1 (aka Hollow Block). So if somebody took away the machine which made them this big, they must have returned it after the war. The more common story is that metals were in short supply and that's why Zildjian made fewer large cymbals.
The expected price is influenced by the Trademark Stamp. Here's my pricing research which is based on completed sales (not asking prices):
http://black.net.nz/avedis/avedis-prices.html
and although I've got a few more 26-ers in my completed sales since that analysis, the only 28-ers I've ever recorded are gongs.
These are the main factors which enter into the predictive pricing equations
condition (photos would help assess this)
weight
production era (trademark stamp plus style of hammering and lathing and bell morphology)
You can check the trademark on my site
http://black.net.nz/avedis/avedis-by-years.html
or you can post a picture here. More pictures equals better price estimates.
The largest diameter I've recorded is 30". That was a 1970s era cymbal which sold for $1500 on 16-Feb-2016. Is that a reasonable expected price for yours? Hard to say on such limited evidence. Based on completed sales for cymbals larger than 23" it doesn't look like larger diameter automatically adds value once you get past 24". This looks like a case where despite being relatively uncommon, they aren't so highly sought after. Perhaps because they are a collectable novelty rather than a bread and butter ride cymbal in most people's minds.
If it is a pre WWII cymbal the expected value would be higher than if it is later.
