Japan Offices In China #21 Or #49?

I am trying to better understand the differences between Japan Offices in China Scott #21 versus #49.

I was biding on ebay for the first image of the used Violet 10y thinking it was #21, but when the prices went higher than I expected I looked further into #49 and the spacing of the overprint.

The second image is #21 from a prior Harmer action for comparison purposes.

Is the only difference the spacing of the overprint ? 6.5mm on #21 vs 4.5mm on #49?
Are forgeries common and easy to detect?
What am I missing?

The used item ended up going for $57 this morning.
Is this is really #49 and the Scott CV is $1,500 (in italics) then not a bad deal
(that maybe in hindsight I should have tried for?)

I look forward to learning more about this area and your opinions.

You are not missing anything.You saved yourself 60$ or more. The e-bay stamp is a forgery.
The genuine stamp is worth more than 1500$ Spacing is wider than the the e-bay stamp.Empress Jingo was the first woman on a Japanese stamp...

Below the real m'coy:

Seeking clarity on the ebay lot here. The overprint is a forgery on a genuine stamp? Both the stamp and the overprint are forgeries? Etc. Please address both parts separately.

The stamp is genuine but with a manipulated cancel.Genuine cancel that has been retouched where the overprnt (China) is placed.
Used examples are very cheap of the non overprinted stamp.

Also, if I'm interpreting Scott correctly, #21 is unwatermarked and #49 is watermarked (#141 -- the zigzag watermark). But I haven't had the time to confirm this with my Japanese catalogs.

yes Philatarium, the watermark is a difference but I could not examine the ebay item (and didn't want to risk buying)

Perf12 - thank you for the explanation and detail. To confirm my understanding, based on your close examination the overprint appears to be a forgery and in doing so the forger also had to re-touch the cancel.
When I zoom in on the image, I think I can see what you are describing and also the overprint appears "on top" of the cancel and just more clear than anything else... cleaner and crisper.
I am interested in what you saw that helped you reach this conclusion.
Thanks again!