The other gentleman is correct, one letter six numbers for the serial number. I don't recall seeing a 6 cylinder in anything smaller than 7-8k capacity and the only Dodge engine in a Yale was a slant six. If it is a flathead it is probably a continental engine. The master cylinder on that vintage would be very common and you could probably match one up if you have a TVH book. A picture of the master could be very helpful for someone who has been in the industry for a while.