This posting drew some efm old-timers out of the closet. It required going back 50 years in time to find someone with knowledge about this. Here is the efm side of this story: Fitzgibbons, located near Center Valley/Coopersburg, PA, produced only the boiler for efm. The S20 stoker was efm's and was installed on the Fitzgibbons boiler. It was the old style, straight-rod, adjustable crank design, which can still be converted to the late model feed-control type.
The efm 350 used the same adjustable crank design. The stoker used in the 350 was the S15 model. The only difference in the old style S20 (520 boiler) and the S15 (350 boiler) was the size of the burner ring. The rings came in 3 sizes and there were S15, S20 and S22 burner rings. The S15 was the narrowest, the S20 is current, and the S22 is the widest. We aren't talking much difference in width; maybe 2 inches between the S15 and S22. All other parts used in the pot and stoker were the same in the 350 and 520. Both models have the round door.
When the DF520 was introduced in the late '70s, the square door was added, and the S20 stoker was used.
All stoker parts are available and interchangeable for the 520/DF520 and 350 models. The narrow 350 rings are not available, but as stated, the S20 ring is perfectly acceptable. The S22 ring is available for those who want to get a bit more btus out of their 520/DF520.
We have all the records of each S15/S20 stoker and what boiler it was matched with; either the 520 or 350. All we need is a serial number and time to look it up, for the original date of sale and owner.
The DF520 boiler uses either a 5 or 7 gpm coil.
efm and General Machine were the same company.
Thanks for jogging our memories.