I am new to keeping Bettas although I've been keeping fish for 20 years. I have been doing a lot of reading online about Bettas for a few weeks and just bought 2 to get started on my Betta learning curve. They are in a separated 30 gallon tank. It is a very mature setup that I was using for other fish and it's a bare tank except heater and filter (with flow turned down and directed at tank glass to reduce water movement).
I am having trouble working out whether one is a male or female. He/she is about 1.7 inches in body length (maybe slightly longer but not 2 inches). Fins are nice but not as long or developed compared to body size as I would expect in a male but more developed than pictures of females I have seen. The other fish I bought is definitely a male, body length similar but fins are way more developed but head and body seem slimmer.
I bought both as males but what got me wondering is the short fin one seemed to have an egg spot (ovipositor), something definitely protruding slightly and has a white tip. You can see it (just) in the 2 pictures below. I put him/her in a large glass jar in the other males tank to see how they reacted for 20 minutes. That's when I took the pics. They flared at each other and he/she got stressed so I put him back on his side of the tank where they can't see each other. You can see in one of the pics he/she developed a horizontal stripe. I know this can be normal for male or female when stressed.
Here are 2 pics of the one in question
Thanks for any advice you can offer