I read your question more carefully
"Will the Jr Gent fountain pen section fit? "
Then realized my answer does not address the Jr. Gent from CSUSA. So, I went to check THAT pen.
The Jr. Gent II currently is only available in a rollerball. They sell a conversion front end for the fountain pen. We don't have any in stock.
So, I checked a tigold fountain pen from months ago and the threading does NOT match. Since they have had several versions, and never changed the name, there is not a definite answer to your question. SOME junior gent II fountain pen sections MAY fit, SOME definitely do NOT fit.
The front fountain sections ExoticBlanks is selling are from Dayacom. THEY thread onto the Dennis properly. And, Dawn has informed me that Dayacom says the #6 nib fits properly. They have better measuring equipment than I do, so I bow to their judgement!!