You are using a stereo cartridge for mono records. This produces unnecessary noise and distortion. Simply wire your cartridge for mono. Series or parallel. —Which ever sounds best. If you use a valve preamp, try wiring the coils in series. The impedance will double and not load your preamp input.
This is about the out of phase noise from the groove walls. You do not want to allow vertical signal from the cantilever/stylus to be transferred. Such movement is noise or distortion that will mostly cancel out with a Mono cartridge. Once a stereo transfer of a mono disc is digitally converted, full cancellation is no longer possible. You will achieve much less noise and distortion if you transfer mono discs in pure mono.
And your transfers WILL be the best on Archive, as you own an SP10 turntable with SME and Ortofon 12" tonearms? The SP10 cannot be beat. It is the best table possible. —Even better than an RCA 72 or a Thorens TD124. I am jealous. Although we use the same tonearms.
Your transfers have less stereo, out of phase noise than the others because your table and tonearms are the best gear possible. The fellows using a lesser tables with a stereo cartridge produce awful 78 RPM transfers.
A 12" tone has less tracking error, which also helps. Your set-up has no stray mechanical resonance I can hear (with a good class D headphone amp and 4 driver FiiOs). The 50lb flywheel really makes a difference. It sounds as good as a table can. Plus you have a modern DC motor with a PLL power supply. Thorens and RCA engineered brilliant mechanical solutions to rumble and wow & flutter, but nothing will beat an SP10 in good shape.
The SP10 is my favourite table. If I was younger and did not already have two TD124s that I completely rebuilt 30 years ago, I'd get an SP10. The SME and Ortofon 12" tonearms are also perfect for 78s, —or ANY record! They have perfect mass and less tracking error. You are getting a darn clean transfer except for the stereo cartridge producing out of phase spurious noise and distortion. Good luck.
PS: Ortofon is now selling an inexpensive high output MC mono standard groove cartridge for less than $200. This cartridge does not need a transformer or special preamp. Check it out!

Foot tapp'n fun. Good sound quality

Matched on Catalog number, Publisher, and first word of creator.
Evidence from multiple sources seem to match
Apr 1945 is from http://www.45worlds.com/78rpm/record/400113