First, the PRR did not have any such classes. Now, they did have H21 and H25, but no H-21 or H-25 😉 Now, if by chance, you've been ignoring Bern Hom for, I don't know, the past 20 years or more, and you MEANT H21 and H25 (BTW, I fixed the subject line), we can address those.
1) The H21/H21A had a large box end sill, while the H25 had one piece pressed steel end sills.
2) The H21 had split end verticals while the H25 has solid end verticals.
3) The side stakes on the H21 had asymmetric tapers with the top taper longer than the bottom while the H25 has symmetric tapers.
4) The H21 is stenciled "H21" while the H25 is stenciled "H25" (usually!)
From: main@RealSTMFC.groups.io <main@RealSTMFC.groups.io> on behalf of Armand Premo <arm.p.prem@...>
Sent: Thursday, June 25, 2020 3:08 PM
To: RealSTMFC@groups.io <RealSTMFC@groups.io>
Subject: [RealSTMFC] Pennsy H-25
Being the Neophyte that I am,what are the basic differences between the H-21 and the H-25 ? Armand Premo