After 50 years of using a Voigtlander Avus (as well as large format, medium format, 35mm and digital SLRs) I'm interested in acquiring a more modern press camera. Graflex / Speed Graphic looks appealing because there so many of them and their parts and accessories, but I'm confused about which one. I have several questions.

I want to use 120 roll film and it appears that the Graflex numbered series works only on cameras with a Graflok back and the RH series works on Graflok and Graflex backs. Is this right? Could I use an RH-12 on a Graflex Miniature Speed Graphic 2x3 Camera?

What about size? It appears that any of these film backs would work on 2x3, 3x4 or 4x5 bodies. So, is there any advantage to a less expensive 2x3 over a 4x5?

Which cameras can use electronic flash?

My interest is in general types of photography.

hey Dan
for the most part if all you wont to use is 120mm film I would just go with a Century Graphic!!

But when you buy a RH- roll film back there are made for the size of camera "IE" one for the 2 1/4 x 3 3/4 will not work on a 4x5!!
and to use a flash your lens needs to have a X on it for storbes

http://graflex.org/speed-graphic/accessories.html

all of the Century have the Graflok backs AND not all of the other Graflex
cameras have them so you have to watchout for that!

Graflex SLR- waist level single lens reflex camera that requires special film holders and Roll Film Holders.
Graphic - hand holdable cameras that take standard film holders, roll film holders, and accessories.

Graphic 22- knob wind 120 rfh, came in versions for 2x3, 3x4, and 4x5 for both graphic and graflex slr cameras, provides 12 2 1/4 inch x 2 1/4 inch (6cm x 6cm) images per roll of film.
Graphic 23- knob wind 120 rfh, in versions for 2x3, 3x4, 4x5 formats for both graphic and graflex slr cameras, provides 8 2 1/4 inch x 3 1/4 inch (6cm x 9cm) images per roll of film.
Versions for Graflex SLR's and 3x4 cameras are uncommon but occasionally show up.

Graphic Rapid Vance rfh come in 2x3 and 4x5 versions.
http://cameraeccentric.com/html/info/graflex_13.html
RH12- 12 6cm x 6cm images per 120 roll.
RH10- 10 6cm x 7cm images per 120 roll.
RH8 - 8 6cm x 9cm images per 120 roll.
RH50- 50 images per roll 70mm film.

Format versions-the mount plate that the shell is on will fit the film gate for the format it was made for. Adapters to up size can be made, down size is not advisable.

Graphic- has two ridges along the right edge, the inner one fitting into the grove in the back with the outer fitting along the edge of the camera back to provide a light trap. Early versions have only the inner ridge.

Graflex-has a grove that the camera back edge fits into to form the light trap, and is wider than standard/graphic rfh.

Other makes of rfh's may fit also.

1banjo and 45PSS: Thank you so much for your helpful posts. I never knew the film backs are designed for a specific size camera. Some film back listings don't mention for which size camera they are designed!
You've provided a lot of information about film backs. It's worth saving.